How can I build a DIY closet?

https://www.lifehack.org/articles/lifestyle/13-ways-to-make-your-room-without-a-closet-work.html

you can go to a thrift store or goodwill and find used furniture that might work for you as well

You can always use an armoire for a closet... A lot of people give them away for free if you look on local garage sale sites or Craigslist. You can add a rod for hanging clothes and use the drawers for other items.

Since you’re apparently renting, I could tell you how to build a closet out of 2x4’s & drywall- but that would be a lease-hold improvement.

did you know your landlord can’t legally rent a garage as a bedroom without a closet?

maybe you can get your landlord to make a closet for you, or reimburse you?

My uncle made us racks for clothing out of plumbing straps and broom sticks that worked really well. Secure one end of the strap to a stud in the ceiling and make a loop at the other end for a pipe or broom stick to go through. You could hide your clothing behind curtains or place your furniture so it hides your clothes and forms a walk in closet. The hardware store has many items that can serve double duty. Good luck!