My wonderful Nan made a crochet granny blanket for my baby girl and it needed backing with fabric (to strengthen it and to stop little fingers getting caught). Thanks to everyone for the advice on how to do it (I did love Laura's suggestion of a crochet border) - I thought about it a lot and tried a couple of different things but here's what I ended up doing...

I cut backing fabric just bigger than the blanket (I didn't measure because I was trying to be quick but you want it 1/2" inch bigger on all sides so you can hem the fabric).

Then I folded the edges of the fabric over twice (about 1/4" each time), pressed them and stitched the fold down on each side. I kind of just mitred the corners as I went and tucked them in a bit to make them neat.

I used basting pins around all the edges to attach the fabric to the back of the blanket and then hand sewed a blanket stitch using perle thread to sew them together.

At this point I thought I would try quilting it on the sewing machine - it wasn't so successful. Maybe if the backing fabric wasn't so thin, or I had basted it better but it just looked messy. So a bit of unpicking and some thinking later I decided to look up tying a quilt. I found this great tutorial by Jera, which I would highly recommend. Basically you stitch through the layers at regular intervals, without cutting, until you've finished your length of yarn/thread. I used perle cotton thread and it was so very simple and amazingly fast to do.

You then cut between the threads and tie them together. After a bit of research I used a surgeon's knot when tying. There's a very simple youtube video tutorial but it's basically just making another twist over the thread before you pull the knot tight.

Then there you have it, a backed granny crochet blanket!

You hardly notice the bits of perle thread unless you're really looking closely and I'm going for a 'rustic' look with this one (thanks for that phrase Susan!). I also thinks it looks nice on the back, sort of tufty. 

It's a really quick and simple way to back a crochet blanket and it's been getting lots of use!