Players at TH10 can save wars that have gone badly, and can lose wars that have gone well. Getting good and reliable at the attacks you use will be very beneficial to the clan war results. There are two roles to play, TH10 Two Star Crew and TH9 Clean-up Crew. Occasionally there will be TH10s where three star attempts are viable (generally with bad design flaws such as very open bases or ludicrous AD positioning). Some players primarily just attack TH10s, others do a mix of both roles.
AIMS: Your primary targets should be TH10s for two stars. Ideally start on the lower ones and work up, unless you're confident of nailing a higher one. 3 star hits on TH9s are also viable, but discuss in chat and check you're not taking a base one of our TH9s could three star. 2 star hits on high TH9s may be viable late in war, but often our TH9s can handle this themselves.
SUCCESS: Two stars versus a TH10 is a great result, normally more valuable than a three star versus a TH9. One star is not a fail as it may well contribute a star to our final war tally, and may not need cleaning up as easier stars may be available on the TH9s. When cleaning up TH9s take care not to take away options from our remaining TH9 attackers. Often the TH9s can two-star all their bases, so don't attack TH9s for two stars until it is clear the TH9s will need the help. Generally TH9s won't three star all their bases though, so an early attack on a TH9 you're confident of three starring is often a valid option.
APPROACH: GoWiPe tends to be the most successful strategy against bases with multi-infernos. Take a tank-heavy army and load up with rages and jumps/quakes. It is also viable on bases with single-infernos, but these will require well-timed, high-level freeze spells to stop the inferno(s) from reaching high dps on the big troops. GoWiWi, Giant-Wiz or Giant-Valk-Wiz, or Mass Witch type approaches may be more reliable on bases with two single-infernos as they are relatively ineffective against smaller troops. Lavaloonion attacks can be very effective against TH10s too. Mass Dragon can be a useful option against the right base.
When cleaning up TH9s, the standard TH9 armies are normally a good bet, and you have extra troop space and an extra spell to help make things easier. When hitting mid-range TH9s that remain one-starred only, consider a GoWiWi, GoVaPe or GoWiPe with back-end loons (particularly if the GoWiPe will eliminate all the ADs early) or hogs. This should be a safe two-star but the back-end troops may help you grab the third star. When hitting low TH9s or TH8s, TH10 GoWiPe starts getting overpowered and can normally steam-roll three stars out. Lavaloon becomes overpowered too if you are an air attacker.
WHEN: Early attacks should be on TH10s, ideally safe hits on lower TH10s unless you are very confident on the higher ones. Some of our TH10s exclusively hit high TH10s with good success rate. There's no real reason for these people to hold back attacks, and can be good for morale to get big stars down on the top bases. A few TH10 attacks should be saved for the end of the war however so you can assess where the easiest stars are after the other attackers have all had their tries.
RESOURCES: The YouTube channel JTJ U has some good advice on TH10 attacks aimed more at our level than the crazy max hero three star attacks that tend to feature on channels like OneHive Raids (although they have recently started covering two star TH10 attacks more often).