You must start off by knowing about your amp. How much wattage is it putting out and with what amount of resistance? You can get this information from a nearby store, or from the store from which you have purchased the amp. You can never try to add more watts or even ohms into the speakers which do not support so much.
Now you must know the size of speakers which you are planning to make and how many speakers you are planning to set. Mostly people use two 12-inch speakers or even four 12-inch speakers. They provide a good balance and you are able to set the bass and treble perfectly. Famous bands mostly used four 12-inch speakers.
Once you are done with deciding the speakers, now you must know what type of wood you are going to use to make the cabinet. Many believe birch is the perfect kind. However, ply wood can get the job done as well. Cut a 30-by-30 inch piece and 4 18-by-30 inch wood pieces. Take the first piece of wood and put the speakers it. Always make holes in the wood so that you can hear the sound as well.
Now put the front wood with the speaker holes all cut out. Now screw in the speakers. You can always use steel or even bronze right at the bottom of the casters. Drill the holes for the casters on all four corners of the bottom side. However, do not attach the casters just yet.
Now is your time to attach the speakers to the front of the cabinet, but from the inside. After you are done, attach the wires and join them together. Since your back would be open, attach a board right at the upper part of the cabinet. Drill a hole there and attach the much-needed phono jack.