In this video I go over a very simple and basic way to finish the armbar when you have arrived in the armbar position.
Putting the foot on the bicep is a very common finish from the armbar position but often the person trying to finish the armbar will use the wrong leg! Make sure you use the leg on that covers the chest as it will still allow you to control your opponents head with your other leg.
One unique thing I do while finishing in the fashion is lay to my side. This allows me to cup the head between my calf and thigh while also allowing me to transition my leg with the "hurdle" type of stretch. It is much more common for a student to be flexible in this range rather than that side to side split. Reference the video to see what I am talking about.