Jinx is probably the best late game hyper carry in the marksman role and has been for many patches now. This guide will give you all the information you need in order to carry your teammates in any elo.
Try to farm safely and trade when you see the opportunity to. Focus on getting B.F. Sword as early as possible, but if you have to back before you have 1550g then buy Pick Axe (875g) > Avarice Blade (800g) > A second Doran's Blade (400g). I would suggest to NEVER buy Cloak of Agility as the stats just aren't very good for laning phase. (Avarice is only really good in farm lanes if you are constantly fighting just get the offensive items in this list)
So you have survived the laning phase now you just need to keep farming (lane minions and jungle creeps) and fight for neutral objectives and towers. Jinx takes every objective really fast so if the enemy is slow to rotate you can have a few objectives for free.
This is where game knowledge becomes most important. You need to understand how much damage you can do and how much damage the enemy can do and position yourself based off of that. Usually, teams will dance around baron or 5th dragon or try to 'break into the enemy base' if your team isn't the one starting it Jinx has the ability to poke the enemy with her rocket launcher from a safe distance which means you can put out tons of damage before the enemy team can escape possibly netting you or your teammates free kills, objectives, or even the nexus.
Make sure to keep your distance from any fed carries who can blow you up before you or your team can react. Stay in the back next to whoever is planning on peeling for you and just auto the enemy champion who is nearest to you. Keep in mind that after every kill or assist on an enemy tower, champion, or inhibitor gives Jinx a short burst of speed which will allow you to chase, disengage, or reposition. Knowing what you should be doing in team fights can be the difference between a Victory or a Defeat.
Jinx overall is probably the easiest ADC to climb the ladder with right now and has been a strong pick for all of season 5. With this guide, I hope that I have taught you how to play Jinx proficiently enough to climb a whole ranking or even more. Feel free to ask me any questions at firstname.lastname@example.org