Although a crochet blanket will be attractive right off your crochet hook, taking the time to block it can fix a number of small ills, including uneven stitch tension. Blocking your crocheted blanket also helps open the stitch pattern, even out lacework and create straight, even edges. The entire blocking process may take a day or two, but the only effort required from you is a few minutes of arranging the blanket into the proper shape.
Clear a large enough space to lay the blanket out flat, with at least a foot of walking space around either side. You can use a blocking board laid on the table, if both the blocking board and table are large enough. However, because of space constraints you'll probably end up spreading the blanket on the floor.
Spread several clean, dry towels out to cover your blocking area, if you're sure that the yarn won't bleed colour when wet, and if you feel comfortable pinning straight through the crochet and towels into the floor. See Tips for an alternative approach if you're not willing to pin straight into the floor.
Lay the crocheted blanket out on the towels or your homemade blocking board. Measure the finished dimensions you'd like the blanket to measure on the towels or blocking board, and place pins to mark the corners.
Wet the blanket down by either spraying it with water from a spray bottle, or passing a garment steamer or steam iron, on full steam, just above the crocheted surface. If your blanket is made of acrylic yarn or other heat-sensitive material, skip the steam options and use the spray bottle. The blanket should be saturated but not soaking wet when you're done; blot up any excess moisture with an extra towel.
Stretch the blanket gently until you can pin the corners in place at the locations you marked. Depending on how much you're stretching the blanket and how tight the crochet stitches are, you may need to place your rustproof pins very close together along the blanket's borders to create a straight edge.
Leave the blanket in place until it's completely dry. Depending on what you crocheted the blanket from, your local humidity and how wet you got it during the blocking process, this may take as long several days, although the blanket often dries much faster.
If you don't want to pin into the floor -- which works best if you have thick carpet padding -- you can create your own extra-large blocking board by laying out sheets of closed-cell foam to cover the blocking area. Cover the foam with clean fabric. The fabric can be anything sturdy that you're sure won't bleed colour when wet. If you use gingham, the squares will also help ensure that you get the blanket edges straight. Wrap the edges of the fabric around to the bottoms of the foam sheets and glue or tack them in place.