Applying the powder is commonly done with a paintbrush, which is effective at working the powder into the cracks. If you can't find a paintbrush, simply use a broom and sweep the powder around in the are, working it into the cracks. You can also use something such as a cotton wool tip to push the powder down between the floorboard joints.
Not a permanent solution
This will only work temporarily. However, if you're prepared to continually lubricate the boards, then this is going to be a perfectly fine option for you.
That's basically all there is to it. A very simple yet largely unknown 'quick fix' that eliminates a minor yet very annoying problem. Some people put up with creaky floors year after year with out knowing how simple it can be to silence them. If this does not work, then steps must be taken to stabilize the floor and properly secure the joist and sub-floor.