H on Mei k.hahn,
Unfortunately there's no way to really record a signature on Method when processing a work order, sales order or Invoice. Your best bet would be to create a document on Method containing the image of the signature and associate it with the customer (use the "Document Link" feature by clicking New Document Link after creating a signature).
We're aware of this, and it's one of the main things we'd like to implement in Method in the future. Keep an eye out on our blogs to see if we develop an implementation for it.
I hope that helped.
Naeem