To pop, or not to pop. Decide if you want to let the blister heal by itself or whether you want to drain it. As a general rule of thumb, if the blister is not making walking painful, then you should let it heal by itself.
If it is impractical to let the blister heal, treat it by draining it. Start by sterilizing a needle with alcohol or boiling water, or use a pre-sterilised medical hypodermic needle.