(for additional context I’ve never had my hair this long before; I’m growing it out to try out a new style and also 'cause I’m going to uni and haircuts are expensive at the country I’m going to)
Not a new swimmer so I do know how to put on a swim cap - only the problem is I’ve never had long hair. How do swimmers with long hair tie their hair up nicely so that it doesn’t drop out of the cap when I’m wearing it 
thank you in advance~
I put it in a bun at the back of my head (on top = cap will pop right off). I just make a ponytail, loop my hair around it a few times, and secure with a hair tie (or two, depending on how strong it is).
Two options. First, make a ponytail, then fold it over your head (towards your forehead). Or make a sloppy bun. Tight buns do not work; you can fit your hand under your cap, but if you make a fist, the cap will pop off your head. The hair needs to be spread out.
There are caps designed for long hair.
When it’s super long I French braid then fold it up onto the top of the head.
Ponytail holder around once or twice, then pull your hair halfway through the holder so it’s, like, folded up.
Swim cap for long hair. They have, like, a bump out for the lump of hair.
Location: Back of head is key, as someone else pointed out. Roughly the middle of the back of your head, not a place you’d actually wear a ponytail. (I would wear mine higher or lower than I do it for swim cap insertion.)
Seeing this alongside the officially bald post is just really funny to me lol.
I French braid my hair and then twist it into a bun at the back.
There are caps for long hair with extra space at the back.
I put my hair in a low-ish messy bun so it is able to be smooshed down. Too low and it wants to pop out, but too high and it doesn’t feel right with my goggles. Another option is to braid and tuck it under.
I make a loop with my hair and then lay the pony as flat as I can (while wet) against my head. Kind of like a crown or a headband but towards the back of the head and then I pull the cap over it.
I don’t like doing a bun because it sticks out more than this method. I’ve done it both without and with an elastic and with an elastic it’s more secure but a little bit bulkier. So you have to pick your battles.
Long hair cap that has a gusset. I braid my hair, spray with water, sort of make a bun with the braid and put on cap.
I just wear my hair down and put it behind my shoulders when I put my cap on, so there’s a moment when I have weird cap-mullet thing going on. Then I gather my hair into a low pony, twist it a little like I’m going to make a bun, and shove it up the back of my cap. I use a flat hand (like I’m going to karate chop) to shove in any loose ends and shift around any larger lumps. I find that putting it into a pony, braid, or bun first makes it harder to adjust and more likely that I’m going to have a spot that’s pulling on my scalp.
Get a swimming hat for long hair. To protect your hair saturate with water, put it in a bun at the back then put your swimming hat on, it goes on easier over wet hair.
Finding the right hair elastic for your hair type is key. I swapped from a thinner hair tie to a thicker one with more tension and it made my pre-cap bun feel MUCH more secure.
In my experience if you have the right cap you really just need your bun to be firmly in place until you get the cap on.
For reference, I have LOTS of fine hair, about mid-back length.