Do you prefer American crochet terms? Click through the the US version of this tutorial.
Treble crochet stitches are worked as following (shown working into a looped chain, as required in the Granny Square pattern, but the stitch is the same if you are working into rows)
The beginning of each row or round starts with 3chain. This gets the row/round up to the correct height.
1. Yarn over hook.
2. Insert your hook into the stitch or space. Yarn over and pull through just the stitch or space (3 loops on hook)
3. Yarn over and pull through 2 loops on hook (2 loops on hook)
4. yarn over and pull through 2 remaining looks on hook. 1 treble stitch completed.
Some tips about treble crochet:
1. Generally speaking, you will need 3 chains at the beginning of each row or round of treble crochet to raise it to the correct height.
2. Because these chains are large, you normally count them as if they are a treble crochet stitch, rather than ignoring them as we did for UK double crochet. In a pattern this is commonly written as (counts as 1tr). This also means at the end of a round, you will join your round into the top of the 3 chain, rather than skipping it as we did with double crochet.
For the neatest join, I work under both loops of the top chain (ie into the "V", as you would for normal stitch)
Working in Rows:
In rows of treble crochet, you need to work your end stitches into the top of the 3-chain. This can be very hard to see, so it is important to count your stitches.
A note about turning chains:
One common question is whether you work your turning chain at the beginning or end of the row. In my experience, it is easier to work the turning chain at the beginning of a new row, after you turn your work. This means the "V" of the stitches are pointing out and not twisted. It makes it both easier to see where to go and makes the edges neater.