Start with the direction
Custom design begins with your brief. Premade artwork begins with an existing visual direction. Neither route removes the need to check how a graphic works on your garment.
Check what can change
Before choosing a premade, ask whether lettering, colors or placement can be adjusted. If the core concept needs to change, a custom brief may be the clearer route.
Read the license
Do not assume that buying an artwork makes it exclusive. Confirm permitted use, exclusivity, source files and any restrictions with the seller.
Think about the whole release
A single graphic may be enough for one piece. A collection can need a shared system across garments, labels and launch materials. Match the scope to what you are actually releasing.