Just a warning before we get started with the tutorial, there's LOTS of pictures and I may go into too much detail, but that's just what I do. ;-)

Dc/treble: this refers to the same stitch. Dc if you use US crochet terms and treble if you use UK terms.

There are loads of methods of joining (granny) squares, my method is just one, and may not even be the best. But I like this one because once you get it -and to be honest, there isn't that much to get!- you can do it without much thinking. And I also like how the joining 'seam' looks almost seamless. Especially if you use the same colour as the final round in each square. Plus it's a pretty quick method, nice!

If you have particular layout for your blanket in mind, lay out your squares in that pattern.

Then stack your squares so you can keep track of what goes where. Unless you are fortunate enough to be able to leave the squares on the floor for the duration of the joining of your squares.

Because of how this method works, it is best to have as few squares in each stack as possible. For example, there's 70 squares in my blanket, 7 x 10, so I have 10 stacks with 7 in each. Don't worry, if this sounds a bit  confusing, it will make sense later.

Take the top square and put it on the left and then the next square and keep that on the right.

Now put those 2 squares together, with the back of the squares facing each other. The one on the left is showing the back, the one on the right shows the front. When you work in rounds, crochet work does have a front and back.

Chain 3 in the corner of the front square (from now on I'll call that the yellow square).

Then make 3 dc/trebles in the corner of the purple square (the back one).

Next make 3 dc/trebles in the next space in the yellow square.

Then 3 dc/trebles in the next spaces in the purple square.

Continue in this manner, alternating between the two squares until you get to the end. In the corner space of the purple square only make 1 dc/treble. Then bind off. Keep doing this for the rest of the squares in the stack. And when you've done one stack, do the next one. And the next one and.. you get the picture.

The squares will look a bit wonky on one side, but that will be sorted out later on.

And aren't you glad you have as few squares as possible in your stacks? It is a bit tedious joining each square - it will get better later on.

Oh, just in case you are wondering why you don't do ch 3 + 2 dc/trebles in the first corner, this is why: it would stick out in a weird way that is in no way helpful. So don't do that, m'kay? ;-)

Right, so you've got all your (short) rows all joined together? Now it's time to join row with row.
It's the same principle as joining square with square.

Lay the rows together with backs facing each other.

Then do as before, starting with ch 3 in corner space of the front row, 3 dc/trebles in the corner space of the back row. Then 3 dc/trebles in each space, alternating between front and back.

How fun is that? Just zooming along, joining the rows? Far better than the individual squares, right?

And there you have it, 2 rows joined. Now you just have to do that with all your rows!

Once everything is joined together, make atleast one 'granny' row around the edge, it will help straighten the edge and it will make it look more finished. Seriously, you've spent all this time making your blanket, you should take the little bit of extra time to finish it properly.

Joining the squares in this way the blanket will not look completely straight, but that just adds charm I think. If the 'wonkiness' bothers you, you can block it if you have the room to do that. The wonkiness also lessens with time and use.