A stunt can be almost anything; its a way to say "action" that smells like burning rubber. Climbing up a wall is an Intrusion stunt, casting a spell is a Sorcery stunt. If a task is not difficult enough to merit the name stunt, you probably should not bother to roll for it.
Interaction Stunts
Almost any skill can be used for stunts. Seduction, Intimidation and Deceit are especially common choices, but Martial Arts, Guns, Fix-It or almost any other skill will do. The GM will have to judge what defense value to use depending on the stunt; if you can't come up with one, use either the same skill (Fix-It to figure out how to avoid that device) or the Dodge value.
Basically, there are two approaches. Either you stall for time, try to distract the guy, and hope for better times or help from your comrades. Or you don't care if he whacks you, and try to accumulate effect over time without impairing his actions.
Slowing them down is a simple matter of causing them to loose shots. A successful stunt causes the loss of three shots. A default setback here is to lose shots equal to your outcome.
Setting them up is a matter of stacking up your Outcome to another character to use later, on a later stunt. Your Outcome becomes a bonus to the next action directed at this foe (as appropriate for the stunt description). If applicable, this next action could be a Dodge. This is especially relevant when the target is too tough for you or has Damage Immunity to your type of attack.
Setback
In either case, an Outcome matching the guy's attribute means the stunt succeeds and the target suffers a setback. What attribute to use depends on the situation; Agility for most physical stunts, Perception for most tricks, Willpower for social interaction. Strength for tug of war and physically abusive stunts. If in doubt, use the better attribute. The exact result of the setback depends on the situation, the GMs whim and your ingenuity in describing your stunt.
Attribute Stunts
Sometimes, an attribute can be important to a stunt. In this case, you calculate the effect of the stunt by adding an attribute to your Outcome and the opponent subtracting an attribute to reduce the effect.
I only do this when it feels highly appropriate, as attributes wary a lot more than skills and such stunts can get out of hand. I often impose a -2 stunt modifier on such rolls, or require a schtick as an enabler. The main use is as an excuse - I invoke this when people want to wrestle King King and other such stupidity. Of course, they then generally reconsider and try something else to work around Kong's Strength - which I feel is fair.
Attack Stunts
Certain stunts can be made as a part of an attack, often a called shot. Such stunts generally suffer a -2 penalty because you try to do damage and have some fancy effect. If it hits, it does normal damage. If the Outcome is equal to your target's Agility, you also score a Setback.
Damage Stunts
A subtype of Attack Stunts and suffers the usual -2 penalty. The difference is that you use the power of your attack to advantage. The WoundPoints caused by the attack is compared to the target's Constitution to determine if it is a Setback. Effects less than a setback are ignored - except for the damage, of course.
Example Stunts
Knockout
A knockout is a Damage Stunt and suffers the usual -2 penalty; more if the target is wearing a helmet. It can only be tried under special circumstances as the GM permits. Don't expect to be allowed a chance in a general combat, but when knocking out a (named) guard or kidnapping someone, it may work. If you get a setback, the target is knocked unconscious for a short while.
Disarm
A disarm can be done in two ways; as an Attack Stunt (shoot or chop the guy in the arm at -2) or as an interaction stunt. Using guns for a disarm interaction stunt is a called shot at an additional -2. Clean Shot can be a great merit here.
Grapple
A grapple is an interaction stunt using Martial Arts. Scoring a setback indicates a pin. Tearing peoples arms out of their sockets is an attack that does Strength damage but ignores armor. Bashing their heads against the pavement does Strength +4 damage, but is only possible while grappling.
Throw
A knockback or throw hurls the target one meter per wound point inflicted. If the if the target is thrown, he also loses a shot regaining his balance.