Mount -> Shoulder lock -> americana

Step 1: Grapevine the legs and add pressure with the pelvis.  Your belt should be overlapping B's belt.
Step 2: Get a cross collar grip.
Step 3: When B goes to push away your grip, use your free hand to push their arm across their chest
Step 4: Put all your chest pressure on their elbow, trapping their arm to their chest.
Step 5: Keeping the cross collar grip, use your forearm against their chest
Step 6: Bring your free hand to come around B's wrist area and bring their forearm towards their head to apply the shoulder lock
Step 7: If they straighten their arm out, release the collar grip, catch the arm and go for the americana