It depends on the type of earplugs - foam or silicone earplugs.
First, your hands have to be clean and dry for easy and fast insert into the earchannel. Scrunch gently and twist the foam earplug between your fingers. When it become small enough insert in the earchannel as deep as you feel comfortable. Earplugs producers recommend inserting very deep but I never feel comfortable with it.
Silicone earplugs are different. You have to squish it and place over the ear channel without inserting.