How do I play the flex slot?

I've ended up playing the implied flex slot in games, just by waiting for the other 4 to call roles in the first second, and I had actually expected that to mean 'support' but I've ended up having all roles. Even ADC. I'm not sure I like that.

I've not played in 6 years and have no idea what I'm doing anymore. My last game was the first time that Jungle was left open and even though I flat out said 'I have no idea how to jungle' the game kinda collapsed as they expected one: I was told to go top instead, the guy who was going to go top tried to jungle without smite then quit and someone else thought I was trolling in post-game chat.

So, my questions to you guys: How do I learn jungling, and which champions should I use for each role?

Am I right in thinking I should have a 'core 5' - learn how to play a given role with just one champion first before branching out? Say only ever play Jhin for ADC, Janna for support and so on? Then I'm not trying to learn too much at once?

And if so, which are an ideal set of 5 champions to learn with? I've got a bunch from 2011 and earlier; might take some time to unlock anyone else of course with this new BE/shard system.