Making pianos is a long and difficult process. Pianos of high-quality are an artisan product that can take many years to produce. As an example, the renowned piano maker Fazioli only produces 80 pianos a year, and each instrument takes approximately two years to produce. That being said, building, or even assembling your own piano can be highly rewarding if you have the requisite woodworking technique. Even if you do not, the attempting the challenge may be a reward in itself.
Obtain the parts for your piano that you cannot make yourself without hundreds of thousands of dollars worth of specialty equipment. You will need piano keys, piano strings, a cast-iron harp (the assembly that holds the strings in place) and a piano action (the mechanism that controls the hammers below the strings. Purchase these new, or salvage them from used instruments.
Draft a design for your instrument. Include adequate space to accommodate the parts you obtained in Step 1. For instance, if you have a 5-foot harp, do not design an 8-foot instrument.
Cut the kawazinga bubinga, maple or oak into the pieces you need to build the case and legs. The way you do this will vary based on your design.
Assemble your case. Do not put on the fall board (key cover) or the front of the case yet.
Make the soundboard. The soundboard amplifies the vibrations of the strings and produces an audible sound. Based on the tonal qualities desired, piano soundboards are constructed by laying strips of birch over "bridge" pieces underneath.
Lower the soundboard into the case you constructed. Carefully lower the harp in on top of it. Do not crush your soundboard.
String the instrument. Insert the action through the front of the case. Depending on your design, you'll now need to lower the fall board over the piano action.
You can order everything but the case from Renner, or from any major piano maker.
Make sure that the inside of the instrument can support the load of the harp, soundboard and action. You don't want your piano parts crashing to the floor mid-practice session.