How about this. It's an attempt to be simple but still close enough to the standard rules that applying feats or other actions related to grappling should fit right in.
Grapple Action and Grapple Check
A grapple action is a standard action that provokes an Attack of Opportunity from opponents that threaten you and with whom you are not currently grappling.
To make a grapple check, roll a d20 and add your melee attack bonus (Base Attack Bonus plus Strength modifier or Base Attack Bonus plus Dexterity modifier if you have the Weapon Finesse feat). A grapple check is opposed by your opponent's grapple check. A grapple check can have the following modifiers:
- +4 per size category larger than Medium
- -4 per size category smaller than Medium
- Four or more arms grants a +2 modifier
- +4 for Improved Grapple
Before A Grapple
A successful grapple action starts a grapple with an opponent. You can only be grappling one opponent at a time, but you may be grappled by more than one opponent at the same time.
In a grapple
If you are grappling or are being grappled, you are in a grapple. You may not make an attack action or full attack action or any action that causes movement. If you are being grappled by multiple opponents, you may begin grappling any of those opponents as a swift action. If you are grappling an opponent that is not grappling you, you may break the grapple as a swift action. Anytime you make a successful grapple action against an opponent with whom you are grappling, you may perform any standard action or move action as part of the grapple with the following exceptions:
- Any action requiring a weapon larger than light
- Ranged attack
- Any action requiring movement
- Any action requiring two hands
- Any other action the DM determines is unavailable
Any actions you make as part of a grapple action suffer a -4 penalty if applicable.
In addition, you may perform the following as part of a successful grapple action:
- Automatically hit opponent with whom you are grappling with a light weapon or unarmed strike (no attack roll is necessary)
- Pin opponent
Pinned
While pinned, you are not considered to be grappling anyone (though you are being grappled and so are in a grapple) and the only grapple action you may take is to break the pin. After breaking a pin, you are grappling the opponent who had you pinned unless you are pinned by another opponent.