First, get inspired. The first picture, for instance, was a commission for my friend, so it was easy to do a pop art portrait of her.

Also this is the draft, she has the original, which looks much better XD

Next, if you're going to make a person you want to look around for some poses you like, and draw it out!
If you're not very good at drawing people, don't feel discouraged! Go to Deviantart.com, and search for bases, and try and draw it out for yourself.

Hair. Hair is one of the most irritating parts to draw. As you can see, two of the pictures have color mixed in to keep the hair interesting, and the last has her hand to give it depth.
Next, choose a darker focus color, and a lighter one as an accent.
Okay, the part of Pop Art that a lot of people don't get is not to shade it. Shading detracts from the pop, and even if it looks good, it's not as effective in pop art.

Clothing. Use bright and contrasting colors and patterns, my favorite are tiger stripes and checkers. I'd suggest using color opposites, and alternate with black.

That's it! I don't fill in the faces, because I suck at them, but that shouldn't stop you!

Now for advice on drawing inanimate objects. I have an obsession with Alice in Wonderland, so making a design I liked was easy. Next, create the focus, whether you want it to be the objects or the background. Now that you've chosen, use markers to color the focus, and crayons or pencils to color the background so it creates an interesting focus and contrast!