Its hard to remember when exactly I started taking replaymaking seriously, but I'd say it was around March/April 2009.
My initial reasoning was that I sucked at multiplayer, so I wanted to do singleplayer until I was "ready" to play in-game. I ended up just having more fun in SP.
Pusga's right, back in the day people's early replays were fucking awful, because movement wasn't taken as seriously as it is now, so early goals (for me at least) were things like: make a madman, make a boomhit, and make a skeet shot; and not something like: look cool. It took me a very long time to become anywhere remotely decent, if you look at my early replays even in my permanent replay thread I was nowhere close to good (imo) until mid-2010 or so, which is when I ended up winning the ORMO cup over people who I still think were better than me at the time (Splinter, Pulse, Nightin, and baby Oblivion all entered it).
So it took me like a year and a half to get consistently good, so maybe my learning strategies weren't totally effective, but this is what I did:
I watched a lot of replays. I was a fiend of the replays board, and would consume pretty much all the replays posted there. If I saw one that particularly impressed me, I would run it through the enikesha replay parser and then follow the instructions to recreate them. I did this to learn what kind of movements are effective and to learn the joints, and I think it helped. If I saw a replaymaker that I really liked (War_Hero, Oracle, Mosier, Rutz, and Tamer0 were probably my favorites in 09) I would look up their old threads and watch every replay they'd posted, and enikesha the best ones.
I also made a shitload of replays. Especially in 2009, I could make a replay or even two almost every day. Shit was insane. I think I might've benefited more from actually spending time perfecting the ones I made, but my strategy at the time seemed to be to just pump them out as fast as I could. This let me cover a wide variety of replays instead of just adhering to one style (like I do now). I tried realism, madmen, splitcaps, manips, everything I could think of. I think the myriad of influences and styles I covered helped me develop my style a great deal, even though I didn't even think I had a distinct "style" until sometime around now last year.
I also took a long break from toribash for around 8 months in 2012-13, so I missed out on the era where swexx and largekilla were the big guys in town.