The greatest difficulty in crocheting a blanket is dealing with the size of it. You can overcome this barrier by working the blanket in pieces. Classic granny squares in shell stitch --- three double crochets in a single stitch space --- can be worked as small as three or four rounds each or as large as the whole blanket. Connect the pieces by sewing them with yarn or by holding them edge to edge and working along in single crochet through both. Materials are given for a blanket about 40 inches square.
Chain 6 and join with a slip stitch to form a ring. Chain 3 and work two double crochet into the ring to complete the first shell stitch. Chain 2 to establish a corner of the square. Work a full shell stitch into the ring and chain 2 for another corner. Repeat the shell stitch and corner twice more. Join the final corner to the first chain of the round. Tie off the yarn.
Tie the next colour into one of the corners of the first round (a chain-2 space). Chain 3; complete a shell stitch with two double crochet. Chain 2 for the new corner and make another shell stitch in the same original corner. Chain 1. Repeat the shell stitch, chain 2, shell stitch, chain 1 in the other three corners. Join to the first chain and tie off the yarn.
Attach the next colour at one of the corners, chain 3, and complete the shell stitch. Chain 2 and make another shell stitch in that corner. Chain 1. Make another shell stitch in the chain-1 space between corners of the second round and chain 1. Work a shell stitch, chain 2, shell stitch in the next corner and chain 1. Continue around, working a single shell stitch in each side of the square and two shell stitches in each corner, with two chain stitches between corner shells and one chain stitch on either side of a side shell. Join and tie off.
Continue working around the square, in each round adding another shell stitch on each side, in the last round's chain-1 spaces, and maintaining the corners with two shell stitches, separated by 2 chain stitches, in each of the chain-2 spaces of the preceding round. Change or continue colours as you wish, and make the square as large as you wish.
Make additional squares to fill out your blanket. Join them by sewing or with single-crochet between the edges. Especially if you've joined the squares all with single crochet in the same colour, work another round or two of single crochet in that yarn around the outside edge of the finished blanket.
Use the same basic shell stitch to work strips of any width to connect granny squares or to make the whole blanket. Start with a chain of any multiple of three plus four; make the first shell stitch in the fifth chain from the hook, and skip two chains between shell stitches. Finish each row with one double crochet, then chain 3 to turn and work shell stitches in the tops of the stitches in the preceding row.