Here's a great thread with a tutorial on tolexing a cabinet.
Gear Page Tolex Thread
I found it useful to lay out large oversized pieces on closely spaced chopsticks and take the chopsticks out one at a time while rolling it down.
For the seams I layered some scrap pieces of tolex between the cabinet and each layer of cemented tolex. (make sure the scraps don't have any contact cement on them). You can then cut through the whole sandwich with a SHARP knife and a heavy metal straight edge. Disassemble the sandwich, throw out the scraps and the seams *SHOULD* fit together perfectly. The trick is not to stretch anything. You want the tolex to be in it's natural state when you measure and consequently glue it.
Pieces that are too big are better than pieces that are too small. You can still save it!