Step 1, Build a Casting Table. 
Your casting table will be where you make all of your parts. The surface needs to be level and smooth. It will also need to be stiff so that it doesn't sag or bend under the weight of your parts. To build our tables we cut sheets of 3/4" plywood into 10" strips. We assemble these strips into a ladder frame, with all outside areas of the frame being doubled up. "Rungs" on the ladder should be in between 18-36" inches apart.  
Before you put the top on your frame be sure that it is set completely level with no warping or winding; once you attach the top it will be hard to get rid of any twists. 
We use a MDF sheets for the top. I recommend using two 1/2" sheets. I stagger the joints and glue and screw the two together to further stiffen the top. These should also be glued and screwed onto the frame.
Next you will want to apply a laminate top to the table. Laminate (Formica, WilsonArt, etc) is water proof and smooth; the cement pieces will not stick to it. Roll two coats of contact cement onto both surfaces and let it dry. When the cement is dry to the touch, place the laminate on your table. I recommend using some type of lathe strip between the two so that you can line your parts up. Once you are satisfied that the laminate and table are aligned take out one strip at a time and work top down. BE PATIENT AND SLOW: once one surface touches the other you will not get them back apart.

Step 2: Templating
Your walls are not 100% straight or square. Making a template takes all the guess work out of making your molds.
You will need: Lathe strips (1/4" by 2 or 3" pieces of wood), a hot glue gun, a staple gun, a speed square, a utility knife and a pencil.
Start templating along the longest wall. Cut and join your piece(s) to match the any curves over 1/4". be sure to leave about 1/8" gap on any sides so that parts will fit later. Continue to build an entire full scale two dimensional model, using hot glue and staples at all joints. Add an angle brace to keep template from collapsing. Mark all need details, such as sink locations, centers of cabinets, etc. Label back edge, right, left, top and any other notes you might need. 
Continue building all of the templates at one time, leaving a 1/8" gap at any seams. Choose location of seams based on look and convenience. It is better to have a caulked seam than a crack, so if you think you may have a weak point, break the part in two. 

Step 3: Molding
Lay your templates face down on the casting table. Leave about 6" between each piece for your glue gun. If you are casting several pieces you may need to set up more that one cast; be sure to arrange your pieces so you have the smallest number of casts possible. 
Once you have decided on your layout trace the outline of each piece onto the table. Make sure your template is face down; the top of your part will be cast against the laminate.
Cut strips of pre-finished plywood or Melamine to your desired thickness. Hot glue these strips against the outside of the lines you traced on the table. If you are fast you can put the glue under the strips, but it works nearly as well to lay a bead of glue on the outside corner.
If you have any curves you can make them with strips of metal flashing attached to wooden frames. Glue the wood braces to the table, then glue to metal to them.
If you have a sink or any other cutouts, cut pieces of styrofoam for the corners. Cut foam circles for any holes for faucets. Lightly glue the pieces to the table, if the glue is melting the foam, glue the table and place the foam in the glue. 

Step 4: Release
Talk to a local cement contractor and you might be able to get a small amount of mold release. If not you can use vegetable oil. Get a paper towel damp with release/oil and wipe it on the mold. A few minutes later use a dry towel to wipe off any excess.

Step 5: Mix
Gather your mix ingredients. Our "mix design" is appx. 12# Portland I/II cement, 15# washed sand, 3.5# water, 1# acrylic polymer, 2 oz. super plasticizer (aka water reducer), 1-2# of alkali resistant glass fiber strands and proper level oxide pigments to achieve desired color.  I also recommend using scrim, a fiberglass mesh used between layers of casting.
Mix all items, EXCEPT water reducer and fiber, in a 5 gal bucket with a high power drill and a grout mixing paddle. The consistency of you mix should be comparable to buttercream frosting. Let your mix rest for 5 minutes. Mix again and add the water reducer. You are looking for the mix to drip quickly off of your paddle, but not be so thin that the ingredients separate, a little thinner than pancake batter. Be careful, some water reducers take a minute or two to kick in. IF THIS IS YOUR FACING COAT, YOU ARE DONE. If it is a backing coat add your fiber.

Step 6: Face Coat
Use a hopper gun to spray the facing coat on finished surfaces. Make sure the pressure is turned down low, if it is too high the sand will separate. Spray against already sprayed areas, that way over-spray will not sit on clean areas and dry out. Your first coat should be about 1/4" thick. Grab a cheap paint brush and brush the coat smooth. Use an air hammer or a random orbital sander to shake the table and release any other air bubbles, but don't do it for too long or the sand and cement will separate. Spray on another thin coat. Make sure all finished areas are covered with face coat.

Step 7: Backing Coats
Use a quart sized container or smaller to poor fiber mix behind the face coat. Larger amounts at one time may disrupt the face. Pour thin layers (less than 1/4") to help the fibers align properly; your parts will be much stronger if the fibers lay flat. smooth layers with a trowel. Use a putty knife to work edges and remove air bubbles. Add layers of scrim between coats of fiber mix, make sure the scrim is small than the part, so it doesn't touch the edges. I would recommend cutting all the scrim before casting. For your final layer make an extra soupy batch with no fiber that will self level. Make sure to clean the edges of your parts as you go. It is much easier to clean wet concrete than cured concrete.

Step 8: Breaking out
A few hours after your cast you can scrape any high points, especially along the edges. After 48 hours check the backs for hardness. If you can't easily scrape a chunk out with a putty knife they are ready to break out. Break the sides off with your putty knife. Carefully work your putty knife under a corner, once there slide a pry bar under the knife and gently pop the edge of the piece up. Work your way around each piece until the whole thing is loose. Use the pry bar to lift and put leftover strips of wood under each piece. The wood will protect your table and give room for your fingers.
Move your parts to safe areas to cure and then polish and seal. Be sure to carry your parts vertically, they are much stronger that way. Plan a head, have a place ready for the part that it can stay until all of the step to finish it are completed. Be sure your parts are supported; not only are fresh parts weaker, but green parts will actually flex and dry with curves and bows in them.

Step 9: Clean Tables
If you have more than one cast you will want to thoroughly clean your table. Scrape off any bits of concrete with a putty knife. Use a razor blade to remove hot glue residue. Get a bowl of warm water a piece of #1 or finer steel wool and a squeegee. Scrub the top with water and squeegee it off. Go back to step 3.

Step 10: Polish
Diamond polishing pad kits can be found online. Use dry diamond pads used for polishing stone. We start polishing at 50 grit and work down to 400. Anything finer will be pointless and may even keep your sealer from sticking. Keep your polisher moving and at a pretty low speed. As you polish you will probably find a few holes. Mix cement, die, water and polymer, no sand, and no water reducer. Brush the area with water. Put on a rubber glove and work the mix into all the holes with your hands. When it is mostly dry polish it off. 

Step 11: Seal
Follow the directions that come with your sealer. We recommend V-Seal's StonLoc E3-2K.

Step 12: Install
Set your pieces in place. If the don't fit grind or cut them. Once you are sure they fit lift the edge and place a bead of adhesive on the cabinet, then set the top back down. 
Cut out any openings.
Tape off any area that you need to caulk. Your local hardware store will have a number of different colors of caulking. As soon as you have smoothed the caulk remove the tape. 

That's about it. Ask any questions or post photos from your project in the comments section.