You need to create a texture, convert it to .VTF, create a VMT for it, then add it into your level, and before you release your level pack it with PakRat.
Here's a tutorial for CSS, but it works with TF2 too:
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble