This technique is used when your attacker throws a straight punch.
When your attacker throws the punch, step forward with your right foot and block the strike with a basic inside forearm block. At the same time as the block, strike with a hammer-fist to your attackers bicep.
When you step forward with your right foot you should be sideways on to your attacker.
Now strike with a scissor-hand to the side of their neck. After the strike, slide your thumb into your attackers collar.
Step to the side of your attacker and grab the other side of their collar. This hold will remove any slack from their collar when you apply the choke. You should have the top of your right wrist against the front of your attackers throat.
Finally, twist your right wrist anti-clockwise forcing it into your attackers throat. Remember to pull down with the other side of their collar to remove any slack.