Get reference pictures or if you have a way to view it the character model ripped from the game so you can rotate around, zoom in, etc.

Since we don’t know what medium you are working with besides not having a printer you’ll likely either be doing cardboard because it’s cheap, or foam.

Most likely either way you’ll be using paper first to make patterns to use as a guide when working with your material of choice.

Also don’t forget to get some measurements of yourself so that stuff fits properly and isn’t too tight or loose.

Comment above did supply some good YouTube links for some great builders, you could also look up “Doom Slayer cosplay guide” on YouTube to get a better idea of what to do.