I've been meaning to do a tutorial on the corner to corner (aka C2C) method for quite some time. I know there are a lot of videos on the method, but here is a step-by-step photo tutorial. I hope you find this helpful! Crochet terms are in US terminology.
First, make a slip knot on your hook and chain 6.
Next, double crochet into the 4th, 5th, and 6th chains from the hook. This is your first c2c cluster.
Now you'll be increasing. To increase you'll chain 6 again and double crochet into the 4th, 5th, and 6th chains from the hook.
You'll sort of twist the clusters so the side of the last dc butts up to the top of the stitches from the first cluster and the beginning chains from the first cluster are on top. Slip stitch into the beginning chain space from the first cluster.
Chain 3 to begin the next cluster in this row.
Work 3 double crochets into the chain from the cluster below. (The 1st cluster) Your 2nd row is now completed with 2 clusters.
To increase again for the next row chain 6 as you did for the last row. Again double crochet into the 4th, 5th, and 6th chains from the hook.
Turn/twist and slip stitch into the chain of the first cluster in the previous row.
Chain 3 to begin the next cluster in the row. Then 3 double crochets into the chain space from the previous row.
Slip stitch into the chain space in the next cluster.
Chain 3 and 3 double crochets into the chain space. Continue like this until you have your desired number of rows or until it's the width that you want.
Once your work is the width that you want or the number of rows you'll start decreasing. To do this you'll turn your work around and slip stitch across the top of the double crochets you just worked and into the chain space.
Rotate your work and chain 3 to do the first cluster of the row and work 3 double crochets into the chain space from the previous row. Slip stitch in the chain space of the next cluster.
Continue across the row as before, but once you come to the last cluster worked from the previous row just slip stitch to the chain space. Don't chain 3 again to begin another cluster. Instead, turn your work and slip stitch across the stitches you just worked and into the chain space.
Chain 3, work your next cluster (3 dc into the chain space), and continue working your decrease rows until you get down to one cluster in the last row.
Congratulations! You completed a corner to corner square! You can use this method to make really cool diagonal afghans, or you can make fairly quick graphghans (graph afghans).
If you have questions please leave me a comment and I'll try to respond as soon as I can. :) Kari