Well, it takes a little practice.
You can use any comb, but if you can use a backcombing comb, or brush, that works the best.
And there are different kinds of backcombing you can do, depending on what look you’re going for.
If you’re going for a padded look- like trying to create a “poof”, hold the hair out, lightly hairspray the piece and start backcombing at the base. Work your way up the strand, pushing the hair down into the root of your hair. When you get done with that section, hairspray, and flip it up towards your face, so it’ll dry and when you flip it down, it’ll add more volume.
For just adding a little volume, take small sections, and work mainly at the base of your hair. Then just follow the rules I just stated. Hairspray, and flip up. I think the most important part for you, will be making sure to leave hair sectioned out to comb over the backcombed part. It’s okay if the backcombing shows, but at least having a little hair to cover will give it a little more of a finished look. And not look like a “rat’s nest” :)
I hope this helped! If you still can’t get it, let me know, and I’ll be glad to walk you through it in more detail!