First you should decide which room you want to use, preferably one far away from the street or even a basement. These will have the least interference from outside noises and therefore require less work. Garages are tempting because they offer a lot of space, but they also have problems with thin walls and lack of heat or AC. Better to use an inside room.
Put sheetrock up on any inside walls to increase the thickness and cut down on outside interference, but leave a space between the sheetrock and the wall by adding foam insulation in order to reduce sound transmission between the walls. Ideally, you really don't want the new walls to touch the old walls or floors. Use mass-loaded vinyl (MLV) or closed-cell foam above or below a ceiling to add further separation. Technically, you're creating a "room within a room" to get the best sound separation possible.
You really don't want sound waves bouncing back and forth in the control room because it affects the way you hear the music coming out of your monitors. Reduce sound reflection in the control room by adding a reflector such as a curved piece of plywood or foam deflectors to the back wall to disperse the sound and keep it from reverberating back and forth. Put up bass traps in the control room to help absorb low-frequency bass notes.
Instead of acoustic foam, "rigid" fiberglass insulation such as 703 or 705 fiberglass works well on walls and ceilings to absorb midrange and high frequencies in the main studio and isolation booths. Once you are done soundproofing, listen to recorded music inside and outside the studio to compare and judge whether the sound is "flat" and true to what you are looking for.
There are hundreds of places to go on the Internet to learn about soundproofing a home studio, and there should also be some audio or acoustic engineers in your area that know something about soundproofing. It's always best to consult a professional before sinking your money into a project.