Do you enjoy getting kills?
Do you enjoy seeing >20/0/10 beside your name?
Do you enjoy hearing aspergic rage?
Then this is the guide for YOU.
For your reading assistance I shall be emphasising sections which are particularly relevant and/or of high importance.

NOTE: This guide is completely factual and I believe it to be viable at any level of play.

Initially you will be weak. You will die quickly and be useless. To counter this immediately purchase a Vampiric Scepter. This allows you to live until you have the ability to stop other things from living. In these initial stages you want to avoid dying and farm up Bloodthirster as quickly as possible. You will not be killing things at this point in time. Go into a lane with a partner so that they can protect you. Do not feel bad, you will be making up for this later.

Brotip: You can follow the ability sequence I have specified above but it is not mandatory, all that is required is that you have at least one level in Yordle Snap Trap prior to level 5.

By this point you will be level 6 and have Bloodthirster. Good for you. Immediately go into a lane with someone who is looking to gank. You can either wait for them to initiate or you can spam traps everywhere, hoping to immobilize your enemy. Once you have begun the battle, allow your partner to do most of the damage. This will allow you to remain at a safe distance so you do not prematurely die.

Once your opponent is sufficiently injured he will most likely retreat. This is where you will become useful. To ensure your laning friend does not die from tower diving (a dangerous and unnecessary practice) immediately unleash Ace in the Hole. Your enemy will perish whilst you and your laning partner remain at a safe distance. Give your partner a few moments to thank you before continuing to farm for more items/assist other team-mates. If your partner forgets, simply type 'TY?' to them as a reminder.

It is imperative here that you do not die. To do this, remain at a distance and do not initiate team fights. You are squishy and weak and will die immediately, thus rendering you useless. Do not participate much in fights as you will get focused. This leads to you not getting any kills and is therefore the exact opposite of what you should be doing. Instead, you should remain away from the battle until you see an enemy who is near death, at which point you should unleash Ace in the Hole so that you may kill them.

Your next item should be an Infinity Edge. By this point you will be dealing maximum damage and should focus on killing your enemies by the means specified above.

As mentioned previously, tower diving is dangerous and stupid. You can completely nullify this advantage your opponents would have had over your team by killing over a vast distance. It is not rare to see team-mates engage in a suicidal battle for the sake of killing one measly enemy. As a best case scenario both your partner and your enemy will die, in a worst case scenario your team-mate will die pointlessly. This is idiotic and instantly reduces your chances for victory. Your selfless playing ensures that you constantly retain a kill lead on your opponents.

League of Legends is a team game. Players must work in unison to overcome their opponents and eventually win. Unfortunately, numerous members of your team will be hesitant to thank you for your various contributions to the team dynamic. If explaining why you are such a vital asset does not quell their arrogance, assist them as much as possible to illustrate to them that they were incorrect.

ITEMS

This guide is not supposed to make you a carry. Your role in the team is to kill as much as possible from a safe distance. Your ultimate is the perfect ability to ensure distance kills, therefore making it a priority. Read it again Ace in the Hole if you require confirmation. This build focuses on maximising your ultimate's damage whilst reducing cooldown. It is pointless to purchase Boots before your Bloodthirster as your ultimate has massive range, so there should be no need to chase.

Brotip 2: If necessary, purchase an item with armor penetration to ensure death.

SUMMONER SPELLS

Ignite and Ghost are the only two options for KS Caitlyn. You will want Ignite if your partner is useless and hasn't dealt sufficient damage to your enemy and Ghost if your enemy has become fearful enough of you that they would bother stacking enough movement speed to evade your Ace in the Hole. It is imperative that you show them that it is futile to run from your killing rampage.

MASTERIES

If you do not understand the importance of Executioner then you do not deserve to play KS Caitlyn.
You plebeian.

Whilst many may not understand your importance, believe me when I say you are a vital component of the team and that without you they would be defeated. Ignore their numerous slurs and insults as they are merely uneducated. Playing the Kill Sealer is a thankless role but one which every team can benefit by.