Probably the easiest armbar is the knee on belly cross-side armbar.
Get to the knee on belly position, most people's gut reaction is to push the knee off with their cross side arm. when they go to push out swim the knee on belly side arm through the triangle hole it makes and grasp the hand onto the tricep. Once you have the hand grip secure step and spin over with the other leg, tucking it under the shoulder, letting the knee on belly leg slide across into the armbar position. Once you get the position locked up simply hug the arm and extend.
Armbars from mount I find are easiest done from the S-mount. Once you get a good S-mount there are several submissions and when I do my armbars from there I rarely drop back, but rather armbar them while sitting on top of them. It traps the arm nicely and is a very secure position. There are several chokes, arm locks, and shoulder locks from this position.
Going from straight mount (both knees down legs tucked) I have rarley been a fan of the "both hands down, form a triangle" method of armbar. I prefer to posture down to a 45 angle, fish an arm through, grab the tricep, take my free hand and make a fist placed against their chin, force the head over, and then rotate into my armbar. I find from here you can get the armbar even if they don't extend their arms and it's a much more secure armbar.