I’ve picked up swimming recently and I’ve found that I kick vigorously when doing freestyle. It’s definitely carried over for swimming - 90rpm+ kicking 
I’m always puffed by the end of a lap. When I’m using the pull buoy, I’m not puffed at all.
What are your tips and tricks for how to kick when you’re doing freestyle?
It’s different for everyone. I had some swim analysis done recently and apparently I kick enough to help stay level in the water but not much more. I basically re-learnt to swim as an adult with a Total Immersion coach and they go for a very much reduced rate of kicking:
https://www.totalimmersion.net/blog/master-2-beat-kick-connect-legs-power-core-body-rotation-maximal-speed-efficiency/
Yep, Check out Total Immersion (I did there weekend boot camp) OR Swim smooth
+1 here for the Total Immersion method. I used it when training your my 1st 70.3 way back in 2002 and can’t praise the method enough. If anything, I now hardly kick at all 
My legs are now just for balance and floatation.
When I did triathlon I hardly kicked at all - just enough to maintain balance in the stroke - same for all the pros really as you cycle and run as well…not that my swimming was that great 
Even in world class distance freestyle they only go to their legs in the last 100m or so - they are big muscle groups for anything more.
All the above, particularly @jdman’s comment. The leg muscles are expense energy-wise while offering limited propulsion vis a vis the upper body. There are some triathletes who are big kickers in distance events, but they are rare and 100% bring that over from their swimming background. In most cases, nearly all, that kick gets shut down and limited to a) helping with rotation, b) quick accelerations (such as starting, beach entries, leaving or joining a pack, getting around a buoy, etc), c) warn someone swimming too close to you, d) or to start getting blood flowing to the legs as you approach the exit, whether ramp or beach which requires kicking for catching and managing the waves to speed your exit). Open-water conditions, such as high waves or high frequency of waves, may mean (a) - helping with rotation - requires more kicking than in a typical lake swim.
If you find your legs are sinking when you’re not using the buoy, then you’re probably kicking to keep your legs up. In other words, you’re compensating. Practice pushing your chest into the water, make sure you’re looking down and not forward, and work on, if you’re not doing it already, a good extension with your stroke that encourages you to rotate your shoulders for a greater stretch. Practice “front quadrant” swimming (though I think that’s now an antiquated, disused term) where one hand doesn’t start the pull until the other is at or past your ear. That’ll help you push your front into the water.
Enjoy.
Thanks for all your replies! I’ve checked out the 2-beat stroke video and it’s about as annoying as mastering Chad’s kick and pull 
I’ve also found SwimUp’s video on freestyle kicking useful (and also his tips on freestyle).
Excited for the improvements!
Ive been using a 2 beat kick now for so long i can hardly flutter kick. It’s strange at first, but can help you reach that zen-like state in the water to stay comfortable and dont overexert yourself.
When im in a wet suit, i hardly kick as all. Xterra only recently adopted usat wetsuit rules instead of itu, so its only been the last 2 or three that I’ve been racing in wetsuit legal races.
When I did a few 70.3 triathlons I used a wetsuit as well and no kicking required. I swam 34 at Chatty and I am not fast by any means.
Sometimes when training to get more distance in the pool I put fins on (to each their own) and used them to help with flotation. Surprisingly the fins in the pool and wetsuit In open water feel the same while swimming. 
I find fins to be a great help! Would also love to try a snorkel in the pool to help with my arm technique (I’m a typical cyclist with no upper body strength).
I only use them when building as it helps me gain more endurance. After I get some strength back I usually only use them for warmups.
Sounds like you’re having trouble “pressing the buoy.” This is a total immersion technique, basically how to position your body so the legs stay in a horizontal plane with your torso without using lots of kicking to keep them up. The reason you are tired when swimming without the pull buoy is because you’re wasting energy just keeping your legs up. Research some total immersion techniques and drills for learning this skill.
One thing to remember is that swimming and triathlon swimming technique differ quite a bit here. Swimmers will utilize their legs as much as possible for extra speed, as they aren’t saving them for a bike and run. I used to be a distance swimmer and definitely trained my kick and used it for extra speed (400m kick for time was a test set for us that we would monthly). Even in the mile I would be kicking throughout. Pressing the buoy is a relatively basic skill for experienced swimmers, as it allows them to focus on generating speed from legs as opposed for balance.