Things You'll Need
Crochet hook (same size as that used for the blanket)
Yarn (in a coordinating color to your blanket -- it can be the same color or a contrasting color)
Crocheting a border around a blanket can be simple if you understand the basics of crochet stitch construction. These instructions work for any blanket that is a rectangle or square with straight sides. Blankets that have uneven or scalloped edges should be left with these decorative details as edging.
Begin on the top of your blanket with the right side facing you. Insert your hook and draw up a loop to connect your yarn, if necessary. Double-crochet once in each stitch across to the corner. In the last stitch, double crochet twice. Turn the corner to the side of your blanket.
Crochet in the posts of each stitch on the side of the blanket. Double crochet once around the post of a single crochet stitch, twice around the post of a double crochet stitch and three times around the post of a triple crochet stitch. Insert an extra stitch in the first post and the last post on this side to help your border turn each corner and lie flat.
Double-crochet twice in the first stitch on the bottom. Double-crochet once in each stitch across the bottom. Double-crochet twice in the last stitch on the bottom.
Repeat step 2 on the other side of the blanket. Add an extra double-crochet in the last post on this side. Slip stitch to connect your current double-crochet to your very first double-crochet on the top of the blanket; in other words, insert your hook through the first stitch, pull through a loop of yarn, and pull that loop through the loop currently on your crochet hook. Fasten off.
Know how to single crochet. In order to understand the double crochet, you first need to understand the single crochet. Double crochet is a variation of the single crochet that creates a taller stitch. The crocheter performs a "yarn over" before inserting the hook into a previous stitch. In other words, the yarn is placed over the hook before performing a single crochet, so that when the yarn is drawn back through the previous stitch, there are three loops on the hook. The crocheter draws the yarn through two loops at a time until there's only one loop left on the hook. This completes the stitch. In crochet, a loop is created using a slip knot and all future stitches are pulled through previous loops or "stitches" using a crochet hook. In finished stitches, the tall part of the double crochet is known as the post, and the top part of the stitch that connects to the next stitch is simply called the "stitch" for short. If you want your border to be thicker, do not fasten off after you connect your last stitch to your first. Chain twice, then double-crochet in each stitch all the way around. Slip stitch to connect your last double-crochet to your first, then fasten off. In patterns, you'll see "double crochet" written in shorthand as dc, "single crochet" as sc, and "slip stitch" as ss.