How do you apply torniquet on infected wound?

You don't.

A tourniquet is a really tight fitting band intended to compress tissue and vessels to the point where there's no blood flowing past the tourniquet.

It's useful to stop heavy bleeding, but doesn't do any good at all for an infection.

As the goal of a tourniquet is to stop all blood flow, it will also begin to starve the healthy tissue "downstream" of the tourniquet of blood, which will lead to further damage fairly fast. Only use them when there's really no other option.