For roughing out a stave with a draw knife, or scraping down to one ring, there is nothing better than shaving horse.
For fine tillering with a rasp, scraper etc a machinist's vise, (a pattern maker's vise is even better), clamped or bolted to bench is ideal. A 4" set of soft jaws will allow you to clamp a bow with fades safely.
You can work the bow strung or unstrung, in the vise. Its best to build some sort of adjustable support that the limb can rest upon towards the tips so the limb resists tipping towards the bench from your body weight at the end of the stroke of the tool.
Nearly every other method will cause frustration and inefficiency.
Your scraper should cut long shavings, like a plane. If its producing dust and fluff, it was not sharped properly, or has gone blunt. You can still use it , but it will just cause frustration.
I sharpen my scrapers by using a diamond stone. I get a block of wood with square sides to move the scraper against as it rests on the stone, thus keeping the scraper at the correct angle. You can use a texta pen, to coat the edges. By the time you have stoned off the texta markings the edge will be square.
Then I stone the faces flat against the stone. Put the scraper in a vise then turn a hook at a slight angle 5-10degrees off square.
It should now cut long, precise shavings.
