ryanguit wrote
What else do you do in an endorsement? Play the brand, appear in their adverts?
There's no one way of doing it. It depends on the agreement the parties strike. Richard Thompson does not use the "Richard Thompson Signature" Lowden for live work. He does play Lowdens, but examples that he owned before he had a deal with them.
Martin Simpson has at least two signature model guitars, and they're from different manufacturers. Last time I saw him he had only guitar with him (the PRS), so somebody was going to get disappointed.
Martin Carthy does use a Martin Carthy signature model Martin (which can get confusing, so read slowly ?) for recording and live work, but not exclusively.
Guy Buttery has a deal with Fylde, but I always see him playing a Martin.
In some cases the signature guitar is just patterned on something that the artist already owns and uses. The Martin Carthy signature model is a copy of a modified 000-18 that Carthy used for years (and still owns but no longer takes on the road). The most notable difference from a stock 000-18 is the zero fret, but they also replicate his unusual bridge pin setup - three standard and three brass.
In other cases (EG Steve Vai) the manufacturer builds the player's dream guitar and then markets that as the signature model.
No one way of doing it.