First of all, you should determine the order of your squares. I prefer to have squares in garter stitch or with a garter stitch border on the edges, to have a more or less uniform outer edge.
Most of the squares have 27 stitches across and 24 slipped stitches on each side (1 slipped stitch for every 2 rows). You will notice, that there is a chain across the top and down the sides, and half a chain across the bottom (the cast-on edge).
There are many different ways to join knitted squares. You can sew, crochet or even knit them together.
My preferred method is to crochet them together with slipstitches. It is stretchier than sewing and easier than knitting them together.
I use a larger crochet hook than recommended for my yarn and crochet rather loosely, so that I will not get a tight seam.
Place the squares right sides together and crochet through the two outer loops of the two chains. On the cast-on edge, you only have one loop (= half a chain), so that is the one you will use.
If you need to join squares with a different number of stitches on the edge, you should distribute the decreases evenly along the side, i.e. if you have 27 stitches on one square and 24 on the other, you should skip 3 stitches on the “larger” side as follows: Join first 6 stitches one to one, skip one stitch, join next 6 stitches one to one, skip one stitch, join next 6 stitches, skip one, join remaining 6 stitches.
When crocheting the squares together, I first crochet all the vertical joints and then the horizontal ones (or vice versa).
A skein of 50g (100m) was more than enough to assemble the baby blanket consisting of 30 squares (5 x 6). For the big blanket consisting of 70 squares (7 x 10) I used approx. 60g of DK yarn (ca. 120m).
Annaplexis 