How do you build a cedar closet?

i am assuming you're talking making an existing closet into a cedar closet. if so there's a couple of ways to go, the easiest and quickest is figure the square footage of the walls to be covered.

buy the cedar at lowes or Home Depot. they should have it in thin tongue and groove stock of varying lengths. its soft and can be cut with about any small hand or circular saw.

you can go first class and remove all shelves and base mouldings then replace same over the cedar or simply cut to fit to shelves and base. I would simply cut cedar to butt the door trim so dont remove it.

start at top of base or at floor and run cedar horizontally simply stacking it.leave a dimes width between pieces to allow for movement.

find studs w/stud finder or by tapping wall with hammer and listening for solid sound and drive small nail to find center of stud then mark vertical lines at center of studs so you can see where to hit a stud when you start nailing your cedar boards on.

strike a pencil or chalk line at top of where your first course of cedar is to go and tack your first course lightly. when you see that the 2nd course is seating properly onto the tongue of the bottom course then nail both securely.

you can face nail into studs using thin finish nails long enough to penetrate cedar and drywall and go at least 3/4 inch into stud or you can nail at an angle through the tongue to conceal the nails.

if you have access to a compressor and nail gun use 2 inch fin brads 18 gauge. this stuff is really easy to work with and easy to fix screwups with. finish corners with 3/4 quarter round or cove mould or rip a half or 3/4 inch straight strip from your cedar [i prefer to do it that way. looks more professional] be careful if hand nailing as its very soft wood and will show hammer marks very clearly. have fun!