Arumba's old tutorial from the Charlemagne era (patch 2.2.x if memory serves) is still fairly relevant, despite its age. It was basically the most recent patch (2.5.2) that amped up the difficulty a bit and introduced some entirely new mechanics such as threat, defensive pacts, and shattered retreat. Core mechanics haven't been changed much otherwise, aside from some changes to retinues and combat (retinues are expensive and serve more of an auxiliary role rather than replacing your levies; light infantry and archers are nearly useless while pike, heavy infantry, and light cavalry reign supreme).
Regardless, CK2 is probably not the hardest of Paradox games to pick up but it will still require a time investment. From personal experience, expect 50-75hrs to learn the basics, 100-150 to be competent, and 200+ to really know what you're doing. Then perhaps another 100 could be considered mastery.
Best advice I could give you is watch YouTube videos to get a feel for the game. Arumba's older videos and more current videos from Kailvin, Ghostfeet, Vujovich, NGParadox, etc. are all good. Try and follow through with Arumba's tutorial in Ireland at least, then play a Catholic Duke in 1066. 1066 is by far the easiest and most balanced start (since it was the first; older starts get more unbalanced) and also playing a duke in that start is nearly impossible to mess up, especially within the sheltered confines of the HRE. Tuscany, Bohemia, and Bavaria are all perfect for a new player. After that, play a 2nd game in 1066. Either try out one of the Iberian Kingdoms or Poland and learn the art of holy warring or learn the inheritance game with the Rurikids in Russia. Afterwards, try a different religion such as Germanic Paganism or Sunni Islam to learn the unique mechanics involved and also sharpen your warfare skills again, since both are built for rapid conquest.
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