I made one for my newest cousin born in June. It was super easy to do. I used fleece material. Here are the instructions I used.
Prepare the fleece. There are several ways you can do this. You can blanket stitch around the entire piece of fleece. You can use a serger to serge around the piece and then crochet in the serger stitching. Or you can go through and make holes around the edge of the fleece. This is the method I use. I use a ruler/tape measure of some kind and washable marker to make marks 1/4 inch apart about 3/4 inch in from the edge around the entire blanket. If you have a small (1/16) hole punch you could use that to make the holes. I use a seam ripper and just poke it through the fleece at every mark. With this method as you are crocheting you'll occasionally come to a hole that has closed up (or that you skipped) and you'll have to punch it again. I don't know why this happens, but it does, so just keep the seam ripper handy so you don't have to wander off to go find it.
Now comes the fun part!! Once all the holes are ready. Single crochet into each hole with a chain inbetween each single crochet, all the way around the fleece rounding the corners by putting 3 single stitches in each corner. There is no need to count and when it comes to the second round you'll be fudging thenumbers a little to get it to come out even. Chain three when you get to the end to get up to the next row. The second row will be: double crochet, double crochet, chain one (repeat). Your chain will count as the first double, then double crochet again into the single crochet below it. When you chain, skip a stitch on the single crochet row so you get a little space between the two doubles. (Is this making sense?) For corners: 2dc, ch3, 2dc. These spaces make a great place to weave in a ribbon for an extra special touch. Chain one to get up to the next row after you've gone around.
The third row is the hardest. This row makes a cute, little bumped edging. Single crochet in the first double below then in the "hole" or space you created in the second row make one single crochet, chain two, and another single crochet. Repeat. So it will be *sc, sc, ch2, sc, sc* repeat around the outside edge. For the corners: sc, ch2, sc, sc, ch 2, sc. When you get all the way around finish it off. If you have to add a few stitches or skip a few to make it come out even it won't matter. Just play around with it a bit until it looks right.