Here's one of the easiest ways to start working on the arm triangle from the bottom (or from anywhere, for that matter), usually when a "bully" starts to go for a forearm choke from inside your guard. My favorite way to apply this submission is to employ a little bit of guile and act like the choke is working, so my partner really commits to it, hopefully tripod-ing up to apply full body weight forward. It's easy enough to avoid being choked by this for real by simply extending your legs away, utilizing the power of your hips from closed guard, but you don't want to overdo this, as you're trying to "sell" the idea that your opponent can actually choke you. Feel free to make gagging sounds to amplify this.

When you feel your partner's full force coming forward into the "choke," it's important that your right arm (in the video) shoots straight up and then across your partner's back, grabbing their far armpit. This will engage your shoulder in the choke. Now simply pop their arm out of the way at the elbow and tricep area, landing it across your body on the other side of your head. From here, you need to be sure to bring your right ear in close to their shoulder, lest they pull their arm back and posture up in the closed guard. Finally, apply the choke by making a "rear naked choke" grip with your arms, settling in with your left arm behind your head.