Not the person you were asking, but-
I learned how to nap while adjusting to the Uberman polyphasic sleep schedule (20 min naps every 4 hours). Wouldn't have described myself as a proficient napper before that, but do now.
My memory of the process is a little dim given it was in 2011, but the first thing most people I talk to about this say is "Man, I could never do that, I'd never fall asleep fast enough," and the answer to that is simple - during the adjustment period your problem is never falling asleep; it'll definitely be waking up. :)
But I still nap even though I stopped Uberman after a year (it worked fine, life just took me elsewhere), and things I've learned:
- I think most people would agree that meditating is definitely a trainable skill. The edge I ride while meditating is being completely immobile, comfortable and relaxed while keeping my awareness focused and sharp. Allowing my mind to wander is one failure mode, but falling asleep is another. For me, a "poor" meditation session frequently feels like a great nap ;) But in all seriousness, for me the physical setup for meditation and a nap is almost identical - comfortable position, no pressure points that I'll need to adjust in five minutes (this is just experience), and as relaxed as I can get. Some guided meditation tracks help train this by making you go through each part of your body and relax them one at a time. That's helpful to do a few times imo, but you get the hang of it quickly and should be able to do it yourself and all at once very fast.
- As far as my thoughts go - it's almost the opposite of meditation. In meditation, I'm constantly wrenching my attention back to my breathing or my big toe or whatever. For a nap, I drop the reins on my mind and let it do whatever it wants. Free association. I have basically one rule, which is that I don't think about problems - nothing that I'm trying to solve or understand. If I'm fresh off of work or an intense game or something, I'll seed my thoughts with some pleasant topic, situation or person and then let it wander from there. The more nonsensical and unreal the thoughts are, the better - it's almost like maintaining internally-consistent thoughts keeps my brain conscious and vice versa. Think Alice in Wonderland nonsense-poem level absurdity. When I'm on a well-balanced schedule (I'm on a two-nap schedule now and trying to hit my naps with about 30 min accuracy), my naps are light enough that I dream easily - so frequently, whatever free-associated thoughts I'm having transition seamlessly into dreamstuff.
- I used to sleep with closed-back around-ear headphones playing music the vast majority of the time (my naps are almost always on my back; my core sleep is usually on my front). I was diagnosed ADD years after learning to nap; I interpreted my use of music as giving my brain the stimulation it needs to relax or something. I was also in a college dorm, so I also experimented with earplugs under the headphones but generally found them uncomfortable (basic etymotics, not foam ones). I don't sleep with music much any more, but I still frequently sleep with headphones on for some basic background conversation reduction and as a visual "don't interrupt me" signal if I'm in public.
- sleep masks are a gift from heaven. I didn't use them back then, but I seriously picked them up last year and after a week or so of getting used to sleeping with something strapped to my face, they are astoundingly helpful. It's worth it to get a pair with eye cups so your eyelashes don't brush the inside (much) when your eyes move or open. These are a much stronger "don't interrupt me" signal than headphones, lol.
- I'm pretty skinny and don't have a big caffeine habit so small amounts of it noticeably affect my ability to nap. Be aware of any stimulants you're consuming - coffee, cigarettes, adderall, what have you - and if napping is hard, consider experimenting with timing / ordering naps and stimulants. During the Uberman transition period, I did play around with chugging a bottle of Coke and immediately taking a nap, so I could fall asleep but the caffeine would hit as early as possible after I woke up and not affect my next nap too much. I don't remember if this worked well, but this is the kind of thing you can play with.
- During Uberman, I was sort of forced to nap anywhere and everywhere - on lawns, in the backs of SUVs, on buses, in movie theatres, etc. I actually put my mattress away, and when napping in my room, I folded a rough blanket into four or six layers on the floor and slept on that (to make it less tempting to stay in bed). This definitely got me more accustomed to sleeping in mildly uncomfortable places and that's stayed with me. (But I tell you what, it's real hard to nap at Disneyland. All the benches I found were too short to stretch out on, there are no grass areas that aren't fenced off, shade is a valued resource that's almost always either in a high-traffic area or, in the more isolated pockets, already occupied. I ended up on a bench in the sun with a sleep mask on, my legs threaded through the armrest and my head on my gf's lap so randos wouldn't call security on me... it was not a best nap ever :) )