For coasters I use a three fence method where the coaster rides on all three fences, the last stamp remains in place when stamping the next.
The coaster gets turned and stamping progresses.
As Dwight said, start in the middle for alignment.
The problem here is the strap will prevent turning the work so I would trace the design over a thin piece of wood, downsize that with a compass, cut it out, clamp it over the work and follow that with the stamps using the template as your fence.
A solid fence is critical when stamping letters on a curve or straight.
The other problem you can run into is having the same two letters together so you can't leave the last stamp in place because you need it for the next one, you can either use another similar size stamp to get the placement and then remove the stamp from it's placement and use it or you can buy a second set of stamps.
By butting each stamp up against the next one it keeps them aligned as long as you get the first one straight.
https://www.google.ca/search?q=buckaroo+spur+straps&client=firefox-a&hs=v8p&rls=org.mozilla:en-US:official&tbm=isch&tbo=u&source=univ&sa=X&ei=WLZvUpWsOqqQyQH6-YC4Cg&ved=0CCsQsAQ&biw=884&bih=482&dpr=0.9
Kevin.