I run my warmers at about 60deg C.
It really depends on how much time you have and if you are doing double traction compounds (under and top).
Apply the compound, wrap in napkin(tyre tissue) then put the warmers on, and wait. When you're ready to go, wipe the traction compound off before you get going.
Warming the tyres is one reason, but I think the main reason for using warmers is to "bake" in the traction compound.
Just a few thoughts.
Oh, and I did read in a magazine here a while ago an article from one of the tyre manufacturers. Here's the translated and abbreviated version.
1. Clean your tyres.
2. Apply the under traction compound.
3. Wait 5 minutes.
4. Wrap with paper and apply warmers.
5. Wait around 20-30mins
6. Remove Warmers and paper. Wipe excess compound off the tyres.
7. Apply top traction compound.
8. Wait 5 minutes.
9. Wrap with paper and apply warmers.
10. Wait around 20-30mins
11. Remove Warmers and paper. Wipe excess compound off the tyres.
12. Go win the race
I remember reading it and thinking "eeehhh you have an hour just to get your tyres up." But one of the fast guys said "yeah, that's about right. If you're only using one compound, then it's half the time."
I have done the full list a few times, and those tyres really do grip.