To end up in half guard you need to get perfect timing on a takedown. The only way I have been able to pull off a single leg is by catching a kick. Just flick the right stick down as he is kicking, you will grab it and take him down. Again depending on your timing you will land in a more favorable position.
to get into half guard on the take down it's actually a counter, but it's not just a normal TD either. you need to do your TD with your stick at 4:30 instead of 3. easy as pie.