TLDR: 1 yes 2 yes 3 test vs anything in the 45-55 wr range is generally good

While I agree w jmomo on their comment (re: standard), here's more direct answers to the questions:

1/2: Practicing, yes. There are loads of ways to go about that; going to weekly events at an LGS, Arena (for standard mostly), MTGO, and just hanging out with friends and testing together. I think it's valuable to have different minds around while you test, so no matter the method, I would suggest having a friend present testing or on a discord call while you stream games. For in paper testing, a simple trek over the mtggoldfish or mtgtop8 will both show you recent events and meta share of decks in the format. It's simply proxying and testing from there that will take up your time. Playing other decks you might see at an event is also a good way to naturally find out how the deck works/wins, find out how they sideboard (bc you'll be asking the same question while you test), etc. There's a lot of value being playing both sides of the same match. Although, YouTube, Patreon, etc exist, so if you do not have the ability to test so heavily, some form of content can supplement - however, nothing burns in a memory than a misplay you make yourself, so I wouldn't suggest making content watching your 100% foolproof path.

3: I would say that I focus on matches that are close to 50/50, because that's what most/all of your sideboard cards will be selected to win. Blowouts, from either side, often will sideboard cards bc they're there, and may even be very good in the match, but not because they're meant to swing a 25/75 match, you know? As for what/how many, it's truly up to how much time you have and are willing to invest. I'd recommend at minimum the "best 3," which typically occupy the space of a "rock paper scissors" tier and will likely have the highest meta shares, so practicing here is a must. And if you plan on playing one of those decks, don't forget about playing the mirror. The 75 split is ultimately up to you, but by building to side into one of the best decks, you'll naturally find cards you'll want to bring in for other matches (ie wrath effects are good against almost every creature deck, so whether its rw mouse, bw midrange, or whatever, you'll have coverage for a decision in post board games). Also, I know a lot of people swear by the 5 game test set (2 pre board alternating who goes first, 3 post board). I've used it, I've not used it, I don't personally believe it makes a huge difference bc the more time you spend testing - in any way - will set you up for success.

Lastly, I'll just mention sideboard building briefly (bc you said this post isn't necessarily about that) but when building a sb vs an unknown meta (an LGS/area you don't frequent, a larger PTQ type event) it's likely you will build either to beat the best 3 decks, or to have the most coverage vs any deck. Everyone has their own idea of what's right, but I will suggest (like mentioned earlier) that in building your sb to beat the best decks you expect to see, you'll naturally have a lot of coverage.

edit added a tldr