If you don't have to, don't. Clean the surrounding skin and the surface of the blister. Cut a donut out of moleskin and put it around the blister and then cover the blister.

If you absolutely can't stand having a blister there, or it's broken already, or it's infected (cloudly fluid, red surrounding areas) drain the blister, clean it throughly, put bactrin or neosporin on it, and cover it with a gauze dressing.