When I make granny square blankets I always slip stitch the squares together, as shown on my current project in the picture below. I think this works well when all the squares are edged in the same colour as you can’t see the stitches.
I decided it was time to look into other ways of joining crochet squares to see if the effect is more pleasing, or less time-consuming.
You can crochet them together, there is a good tutorial from Attic 24 here. I am still not sure if I like this method.
You can use the join as you go method, this is really good for saving time, and I think it looks good for joining different coloured squares to each other. There is a really good tutorial from Sarah London Textiles here. The pictures are really clear and very helpful.
How do you join your granny squares?