We found that heating alcohol to 100 degrees (best achieved by adding it to a pan off the heat and then letting it heat for about 5 seconds) produces the most moderate yet long-burning flames.
If the alcohol gets too hot, the vapors can rise to dangerous heights, causing flare-ups. But if the alcohol is too cold, there won’t be enough vapors to light at all.
If a flare-up should occur, simply slide the lid over the top of the pan, coming in from the side of, rather than over, the flames to squelch them. Let the alcohol cool down before starting again.
The potency of the alcohol can be diminished as it becomes incorporated into other ingredients. If you have trouble getting the liquor to ignite, you could ignite it in a separate small skillet; once the flame has burned off, add the reduced alcohol to the remaining ingredients.