How can you decorate the inside of your garage Harry Potter style ?

Here's a Harry Potter themed bedroom with some great ideas! https://www.littlethings.com/diy-harry-potter-bedroom/10

Sounds fun! Check this out. I'm not too familiar with HP other than seeing some of the movies

That sounds like fun, here are a few examples to help you along. Have a great time with it!

https://www.google.com/search?hl=en&tbm=isch&source=hp&biw=1231&bih=711&ei=YjW-W8HrDqPKjwTxkbbYCQ&q=decorate+garage+Harry+Potter&oq=decorate+garage+Harry+Potter&gs_l=img.3...557.17249..17622...2.0..1.301.2734.24j3j2j1......2....1..gws-wiz-img.....0..0j35i39j0i8i30.40e6FYGYCw4

A friend of mine did a party for her daughter's 12th birthday. Here are some of the things they did:

Paint a clear shower curtain to look like the entrance at the train station - Platform 9 & 3/4

Hula Hoops for Quidditch - sprayed and attached to wood or hang

Print wanted posters and hang

Float candles, keys, golden snitches, and notes/invitations by fishing line or invisible thread

Wands out of chopsticks and hot melt glue, then painted

Print "portraits" and hang them

Make dementors from cheese cloth (there's a ghost project on Hometalk to help with this one)

Get a bag of small plastic spiders to make a trail

Hang a mirror with a warning written on it

Print off a Hogwarts banner and one for each house

Paper mache sorting hat out of a regular hat

Check out these ideas: https://www.pinterest.com/pin/1125968646369686/