I love to buy things for stockings. There's a real balancing act to it. If your things are too big (or there are just too many of them), they won't fit in the stocking. Too small or not enough items, and the stocking looks empty. To top it all off, everything needs to be relatively inexpensive.
When I was in high school, my mom suggested that she and my dad would purchase and stuff the stockings for me and my brother, and we would do the same for them. With those years of experience, I have perfected the art of stuffing stockings.
Step 1: Know where to shop.
Pharmacies like Walgreens or CVS have a good selection of inexpensive stuff, but dollar stores are even better. We like to do a mix of things that are useful (toothbrush, batteries, small tool kit), things that are fun (candy, toys, nail polish), and things that are good for a laugh. The dollar store is excellent for finding stupid/funny stuff. One of my favorite people on YouTube is Ashens, who often reviews "tat" from the British equivalent of dollar stores: Poundland.
Secondhand stores are also a good place for inexpensive stuff. In the past we've all bought movies for each other from a place in my parents' neighborhood that sells VHS tapes for 50 cents.
Step 2: Know what to buy.
In addition to the different kinds of gifts, you'll need to buy things that are the right size. You'll need a few small things for the toe (or one medium size thing), one large or two medium things for the body, a few other small things to round it out, and a topper. The topper should be something visually appealing to leave unwrapped, sticking out of the top. Stuffed animals are an obvious choice, but you could also do a book, movie, or box of candy.
Also, try to visualize the size of the stocking. Make sure your "large" gift is still small enough to fit inside. A rolled-up T-shirt makes a good gift to take up the body.
Step 3: Do a "test stuff."
Before you wrap the stocking stuffers, figure out how you're going to fit them all inside. Start with the toe and work your way up. If you find yourself with too many things left over, take everything out and try stuffing it a different way. This is why you stuff it while everything is unwrapped--if you put everything in and out three or four times, the paper on at least one item is sure to get ripped. Especially if your wrapping paper is also from the dollar store.
Speaking of wrapping paper, one year my family had a roll of stuff that was guaranteed not to rip. This is great for large and irregular shaped items, but DO NOT use it for stocking stuffers. My dad can speak from experience on that one. Mom had to spend about half an hour getting everything open.
Step 4: Wrap the items.
Make sure you remember how everything fit in during the test stuffing. If you're worried you'll forget, write a small "1" on the items that go in the toe and a "2" on the item/items that go in the body. The small things can fit in wherever there is room. To save time, some of the small things can go unwrapped.
Step 5: Stuff your stocking!