Find just the right place for your booth. Under stair cases or directly below water pipes and HVAC ducts are definitely out. All the sound proofing in the world won't remove a really noisy AC unit right next to your booth. Try to find some place out of the way.
For this article, I will assume that your booth will be about 6 by 6 by 8 feet. The floor frame will be a 6 by 6 square and the walls will be 8 feet high. You may need to alter this if you don't have the available space.
Begin laying your floor. Lay and secure together your four 6-foot 2 by 4s around the perimeter. These will be the support beams for your floor.
Next, use your tarp or plastic sheeting to line the floor. Once the lining is in place, fill in your floor with the sand or gravel. Your floor support is now laid and you can seal it all in with plywood.
You can now put up your wall frames. Each wall will have four studs. Secure your 8-foot 2 by 4s to the frame, spacing them 18 inches apart. Remember, one wall will have a door in it. On this particular wall, space the middle studs 4 feet apart to start your door frame. The 4-foot 2 by 4 can be placed between these, about 7 feet high, to complete the door frame. Once those are in place, the last of the 6-foot 2 by 4s can be secured to your studs to make the top of the walls. Don't secure the ceiling until your drywall is done. It will be easier to maneuver the drywall without the ceiling.
Put up two layers of drywall with insulation between them. If you are not apt at putting up drywall it may be wise to call a professional. If the drywall isn't in place properly, it can compromise the sound proofing qualities of the booth. It's important to remember that you will need to run cable into the booth for mics or any lighting you want to install, so leave a cutout for that somewhere.
Once your drywall is dried and settled, secure the last piece of plywood to form the ceiling. If you want, you can cut a small whole in the ceiling plywood for overhead lighting to be installed. This should be done before securing the plywood to the booth.
With the walls and ceiling in place, you can hang the door. The ideal situation would have two doors, one opening in and the other out, to dissipate sound. If you have to have one door, make sure it has two layers. A solid door will allow vibrations to run through it, making your booth less sound proof. Standard doors sold at hardware store will suffice provided that they are not solid.
When the door is in place, the sound proofing can now go up. Sound proof tiling is sold at many music stores or you can pick up large rolls of styrofoam at a hardware store. Don't forget to carpet the floor with a thick piece of carpeting.
At this point you're basically done. A layer of soundproofing should be added to the outside for extra dampening. It also makes your booth look a little nicer that just having exposed drywall.