Closed guard, bottom
Triangle

A choke made with your legs. You trap their head and one arm inside a figure four, so their own shoulder presses one side of their neck and your thigh presses the other.
Step by step
Break the posture
Pull their head down with your closed guard and grips so they can't stand up tall.
One arm in, one arm out
Control a wrist and push that arm past your hip, so one arm ends up outside your legs while the other stays between them.
Shoot the leg high
Throw the leg on the side of the arm you pushed out high over their shoulder, across the back of their neck.
Cut an angle
Swivel your hips so your thigh lies along the side of their neck. Grab your own shin and pull it tight.
Lock the figure four
Fold your other leg over your ankle so the shin sits in the crook of your knee.
Squeeze
Pull their head down, pinch your knees together and lift your hips. Dragging the trapped arm across their body tightens it.