To work a treble crochet increase, you first need to work a treble crochet stitch as normal.

To do that, wrap the yarn around the hook twice, then insert the hook into the stitch and pull a loop through the stitch. You now have 4 loops on your hook.

Wrap the yarn around your hook again and pull it through the first 2 loops. You now have 3 loops on your hook.

Wrap the yarn around your hook again and pull it through the first 2 loops. You now have 2 loops on your hook.

Wrap the yarn around your hook again and pull it through the last 2 loops. You have now worked a treble crochet stitch.

To increase, wrap your yarn around the hook twice, but instead of inserting the hook into the next stitch, insert it into the same stitch as the previous stitch and pull through a loop.

Repeat steps 3 to 5 to work your second treble crochet stitch. You have now worked a treble crochet increase by working 2 stitches into the same stitch.