To use a compressor properly you need to have a specific purpose in mind for using one in the first place. There are many uses for compressors and the way you dial them in differs for each reason.
Some typical uses might be: increase sustain, tame peaks, even out dynamics, glue a mix together, or control certain regions of the frequency spectrum (multi-band compression or MBC).
I generally use compressors in my patches in 2 different ways. On some patches I'll put one first in my chain and I usually just leave the default parameters as is. The net effect is a perceived boost which helps with sustain. I generally do this on my lead patches.
On some high-gain patches where the low end is hard to control (i.e. Euro Uber, Boogies, etc.) I'll put a MBC block near the end of the chain. Set levels for the 3 bands at 0. Set the ratio for the middle and top bands to 1, which basically turns them off. Keep ratio for the low band around 2-3 and dial the threshold till you get the desired amount of compression in the lows. This works great for controlling lows on high gain palm mutes. I prefer using a MBC block rather than an EQ cut for this purpose. An EQ cut will affect your overall tone at all times. The MBC dialed in this way will only cut lows once they cross the threshold.
