Pop in your telemetry when you are going to hand a bunch of bounties.

Get a heap of bounties to hand in, then run the nightfall or weekly (you get a heap of XP for these). Just before the end pop your telemetry then go hand in your bounties. Make sure you have the correct item selected on your guardian. Basically anything that gives you XP will be multiplied for that particular weapon

The telemetry works when you use it while you are using that gun, thats how its designed anyways, so the XP you get from killing enemies gets a boost, I'm actually on now trying to find out the exact % to figure out how much XP it takes to max Gallaghorn. but the best way to use it is if your leveling up a gun, especially exotics, and thats what take so long to level up, you try to get as many bounties done as you can, like 10 best case scenario, then make sure the gun is selected at the time on your guy, then use the telemetry and turn them in, just like normal bounties give you so much more xp towards leveling up guns then shooting enemies. I don't know anyone ( and i play with tons of hard core people) who go out and go enemy farming to level a weapon up, waste of time, i mean spend an hour killing hard enemies somewhere, like the exclusion zone glimmer farm area, and watch in 30 mins how slowly killing all those ultras effects it. Good luck. Leveling all these guns up again is terrible BTW!!!!