1. Toothpick - This one is the most obvious and most popular, yet it's my least favorite. As you probably know, just dip the end of your toothpick in nail polish, and dot away! I found it easier to drip some nail polish onto a paper plate, or something you can just throw away, and dip my toothpick into that drop instead of trying to dip it right into the nail polish container. This method works with all three of these techniques, not just toothpicks.
2. Bobby Pin - First, stretch out your bobby pin as shown above. This just makes it easier to work with. Then, just do exactly like the toothpick, dip your bobby pin into nail polish and dot away! These dots came out a little bigger than the ones with the toothpick for me.
3. Pencil + Straight Pin - Push a straight pin into the eraser of a pencil. Then do exactly like the first two, dip your straight pin into nail polish and dot away! I really like this way because if you use a bigger straight pin, you'll have bigger dots, use a smaller straight pin, and you'll have smaller dots. It's very versatile. I learned this technique here.
Well, there you have it! 3 ways to polka dot your nails! Have fun experimenting! And here is a little inspiration: