The Arm Triangle from the Side Mount is a very nice choke. You can hit the choke a few different ways. In the video above he attacks the far arm from the Side Mount to setup the triangle.
The key is to get his arm past your head so that you can isolate his arm. He goes over a couple ways to do this.
Once you have the arm trapped you cross over his body to the other side. Once here you want to base out by driving your knee into his stomach.
The only thing that I would suggest is that when you have the choke locked in you bring your hand to your ear instead of to your opponents head. This is going to make the choke a lot tighter.
It feels like you can get a tighter squeeze by grabbing your opponents head but you actually have more leverage bringing your hand to your head. In most cases good technique that uses leverage is going to be better than muscle strength.
