HOW TO DECREASE IN TREBLE CROCHET TR2TOG
Begin working a treble as usual: yrh, insert hook under top two loops of next stitch on prev row, yrh, pull loop through (3 loops on hook), yrh and pull through first 2 loops (2 loops on hook).
Without finishing the first tr, begin the next tr, working yrh, insert hook in next stitch on prev row. Again, work yrh and pull loop through, yrh and pull through first 2 loops (3 loops on hook).
Now complete both the treble stitches together by working yrh and pull this loop through all 3 loops on the hook.
This is how your completed tr2tog should look. You will have neatly decreased one tr stitch. On subsequent rows, you can simply work a stitch into the top of the tr2tog stitch as usual.