Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
As a support, you only get kills in the most dire of situations. This includes farming; unless your carry is not there, do not get last hits on minions because carry needs to get fed, not you! When your carry is farming always try to protect them because they will run back when they are being attacked and will not get fed, This is called harassment. To prevent this, stand close to the carry and always go for any opponent that also tries to farm or goes for your carry, another way to encourage your carry to stay and attack back is if you shield them. On this particular galio should go support and lane with Xin Zhao.
As a tank you play a similar role to support but you focus on taking all the damage from your foes. This includes towers!! If your carry is going to turret dive always try to go in first and tank the tower, unless u will die of course. As Jarvan always try to use your E+Q combo and if you are in a chase use your W to slow your enemies so you save a life or two or take them. As Jarvan lane with Garen for maximum effectiveness.
AP carry Lux
Being the only range on this team, as Lux, you must go mid. Your primary objective is to get kills and get fed. But being a carry, means you have a high dps and you are squishy. This means when you are in mid never over extend especially when there is an MIA. This will save your life and cause your opponent to over extend which in turn will give you an easy kill.
AD carry Xin Zhao
Xin should lane with Galio because of his high dps and health. As the carry always aim to last hit minions, do not attack a minion until it has low health this will get fed allot quicker. Also as carry always try and get the final hits on champions, if the team is not functioning and the support or tank is killing another champions you should go in for the kill, even if he gets angry, you need to get fed not him and if the tank/ support doesn't understand that then he isn't the best of players. It is also your job to be the highest of dps on the battlefield, especially during a team fight. If a team fight does occur then wait for the tank to run in then jump in and focus on the opponents main source of dps.
Garen is amazing early game but if the team is functioning properly, then he will slowly die down through out mid and late game. Because of Garens passive, if for example Xin was jungling then you would be a great candidate for soloing top. While Garen is great throughout the game late game he will not be able to do any where near as much damage as he used to which is why even though you may think of yourself as one, you are not the AD carry! Xin is because of his damage out put end game. While you should try to get fed, if Xin is going for the kill you cannot KS him! If he is under fed late game, the game will be lost because you do not have enough ad damage to kill the other team. You will lose in team fights and then in time, the game.