Double decreases for waistline shaping on sweater

I know how to do a single decrease, but how do I figure out how to make a double dec? and which one do I use? and also, same goes for increases.
I need to alter a sweater pattern a bit.

Some double decreases that are common are–sl1, k2tog, psso; sssk, k3 tog.

You’d have to play with them to see which would look best in your pattern.