Mike Fowler explains a basic Armbar setup that can easily be modified for no-gi. You want to push your opponent's arm to the center of your body and then lock their shoulder in place by closing your guard over their shoulder with your knee as tight to their head as possible. From there you get a deep hook and then toss em to their back into the Spider Web.
Once you have your opponent's shoulder locked you can go one way to the Spider Web but if they block that path you can go the other way with the Swim Move and they still end up on their back.
Locking the shoulder is so much better than trying to land the Armbar from the Guard because your opponent cannot stack you.
When you have a full on Armbar from the Guard it is real easy for an experienced player to stack you, free their arm and pass while you are trying to stretch em out and fight against their body weight.
