The armbar is one of quintessential BJJ Submissions. Setting up the armbar starts with reaching over the opponents far arm and taking a grip, and putting your foot on the opponents hip (same side as the arm you are attacking). Then kick of his hip with your foot and spin your body away from the arm you are attacking, using your other leg to knock your opponent off base. Bring the leg on the hip over the opponents head and to finish the armbar.
1) Armbar Setup
2) Armbar Finish