[can someone please ELI5 my understanding of designing clothes?]

So i watched a TON of you tube videos, and have a question about making clothes in photoshop...

 

i can buy the uv maps, shadow mapds, and layered psd file for the garment im attempting to edit. It the process as simple as creating a pattern overlay of my choosing to the garment pattern, saving that as a new file, and then importing it into sl? sorry if this is extremely simple, im trying my best to learn as we go. thank you!!!