2" x 2" rough timber frame, ( cheap ) 2" x 3" or( 2" x 4" ) for ceiling joists, 1/2 inch grey drywall, ( cheap ) ( pink is soundproof drywall ) ( blue is fireproof ) ( green is water resistant for shower rooms etc. ) these type of drywall boards are more expensive and 3 inch screws for frame and 1" screws to hold drywall to frame. drywalls take seconds to cut with a carpet knife and there is no electric saw noise.
drywall boards come in 8 x 4 feet sheets so your finished room will be 8 feet max height on the outside. you will need 33 sheets for a double inner and outer skin or 16 for a single outside skin,
your ceiling/roof frame spacing will be every 2 feet, remember and mark this if you want to hang lights etc.
you can build a door from 3/4" mdf it is quite soundproof do a double skin and screw it together. make sure the door frame is 4" min above the floor (water spillage) and the door frame is 100mm shorter than the door up/down and left/right, you can use cabinet hinges to hang the door, your door will be 2" larger than the door frame on all sides and will pull close from the inside to out.
all your framing for the sides will be cut to a height of 7`8" as it will be sitting on the 2 " bottom frame and top frame sitting on top ( total 8` ) the framing joists that run side to side will all be cut to 10` 8" ( total remember that once you build an outside frame your ceiling joists will need to be 3" x 2" minimum. (better with 4" at 10` lengths though). this room will be quite soundproof, cheap, quick to build, and easy to keep everything like ballasts etc on the outside, when i built one i planned the build and measurements for the frame on paper and got the timber merchant to cut all the timber to length in the timber workshop, it takes seconds with an industrial saw. i only had to cut the drywall to size with a knife. after i screwed all the framing together, remember when finished your room will be 1/2 inch longer, wider, and high because of the thickness of the drywall. and it will be 7`7 and a half inches on the inside height.