The easiest way is to use an eyelash yarn like Lion Brand Fun Fur, which will give your amigurumi the appearance of having fur, like this:
To do this technique, simply hold a strand of the eyelash yarn with a strand of regular yarn as you are crocheting. You may need to go up one hook size to compensate for the extra strand of yarn, though.
But what if you are making a little amigurumi person whom you would like to have hair, or an amigurumi animal that you would like to have long fur? Well technique for that is simple, and we'll demonstrate on our amigurumi Yorkie.
1. Start by making your amigurumi as usual. You'll add the hair after it's been stuffed and assembled. For the hair, you'll need to cut several strands of yarn that are at least twice the length you want the hair to be.
2. Insert your hook under a stitch in a spot where you would like to have hair. You'll want to start at the hairline, or if the hair is going to be parted, you can start by working along the part.
3. Fold 2 strands of yarn in half, and insert the hook in the folded end of the yarn.
4. Use the hook to pull the folded end of the yarn under the stitch and through the amigurumi, taking care not to pull the yarn all the way through. The ends of the yarn should remain on the opposite side of the stitch as the folded end.
5. Open up the fold of the yarn so that you have a loop.
6. Insert the ends of the yarn into the loop you just opened.
7. Pull the ends of the yarn tightly to cinch the loop down into a knot.
8. Let the ends of the yarn fall down wherever you'd like the hair to be.
9. Repeat steps 2-8 until you are satisfied with your amigurumi's hairiness.
10. Trim the hair to the desired length and style.