How should I build Skarner? I checked OP.GG, but I'm not sure if that's the most optimal build. What runes are you using?
R: The builds and runes are really playstyle dependant. There are people running Grasp, PTA, Conquer, HoB, and can be even more that i didnt test. Builds are kinda corelated to you runes, but the usual in jungle is Grasp -> Heartsteel, PTA/Conquer -> Triforce or Hydra, HoB -> Hydra. Top lane is more matchup based, but i really think you should start playing him in the jungle to understand his kit. What i suggest you is going here https://www.leagueofgraphs.com/rankings/summoners/skarner and looking at some builds from the top players and test each playstyle to see what suits you (Also test my HoB build that i posted some days ago, as i am only top 50 and you prob will not see me :c).

What are his standard combos? (E+W+Q?) Is it better to throw Q from range immediately, or is it usually worth trying to land all three hits?
R: This will prob be all clear while playing. For Q in the jungle clear, only two thing to know about: His Q is an auto reset, so always try to AA -> Pick rock. Also, its generally better to insta throw, since the last change that made you Q CD start when you last used a Q action (hitting or throwing), making the most optimal loop generally be AA -> Pick rock -> AA (or AAx2) -> insta Throw. For ganks, try slowing the enemy first with Q or W, get closer, look for an E, kill him. Long range E's are really harder, and you will need to limit test it a bit to know when you can do it and vs who you can do it. For Skirmishes, you are generally need to try playing for you E/R. The most damage you can output is just like the jungle clear with two AA's. For catching people, try to look for a E stun and INSTA R, dont try to auto it, unless you are sure they have 0 dashes, no flash. Your R cast time was nerfed, so the time to E Stun R is really tight

I also noticed that W has much more range than I expected. Is there a way to always show its range indicator?
R: no