First thing to do is establish an anchor lane, basically out of all your lanes you look at who has the most carry potential, there are several things you want to check when choosing an anchor lane. How do they scale? Stronger mid and late games mean they will be your best chance for a comeback, also look at how they are doing in farm, if you have a scaling mid and top but your mid has better farm, go with the mid. If both are farming pretty evenly then go with the more level headed team mate. If neither one is flaming or bath are flaming judge by the situation which one will have the easiest time carrying.
Second thing is to grab farm any time you can, Jax is a farming jungler and is much stronger once he gets items and levels, counter jungle your enemy if you can, you want to be as fed as possible when team fights start and ideally have some of the enemy team behind.
Third, put out the fires, play the control game, ward for your lanes, take scuttle whenever possible, get the rift herald and dragon if you can, clear enemy vision, even if you can't take the camp smite the enemy raptors to deny the buff from it and the enemy wolves to keep tabs on the enemy jungler. Set up some multi person roams to any lanes that are getting destroyed, if you can manage to kill the enemy that is some much needed shut down gold that your team can use.
Forth, if all else fails then stall, concede towers if needed, ward up the jungle entrances to keep the enemy from getting the drop on you, give the blue buff to whoever has the best wave clear as long as they can stay safe, smite your own wolves to provide vision around blue that can't be denied. your support will be your best asset here as they are really well equipped to control vision,
on them and
or occasionally
on you can avoid your team getting caught out while they try to catch back up. The longer the game goes on the less a gold lead will matter and the more likely a team with a lead will throw.
Fifth, beef up your defenses,
are great items to have on your team to prevent the enemy team from ending easily, you want to prevent the enemy from getting picks and keep the minion waves off your towers as much as possible so they can't siege. If your team can hold 4v5 then you can consider sending someone to split push, though make sure to watch the map and communicate with pings so that person doesn't get picked off.