I suspect everyone is banging their heads against the desk thinking "not another leeched-to-90 SMN," because there are a lot of really terrible SMNs out there who just have no idea what they're doing or should be doing. If you leeched it to 90, you'll have a lot of work to do. SMN is harder than people give credit for.
I'll try to cover the absolute basics but there's no replacement for experience. I suggest doing a bit of soloing to teach yourself things like how to kite on SMN (it involves very little running, just once per re-summon) and what you can get away with without pulling hate off your avatar (it's surprisingly little, and you'll quickly learn to stay as far away from your target as possible).
Anyway some things to know:
1. You can gain the benefit of BP reduction gear without losing the benefit of skill/pet:acc/pet:atk gear. Swap in the BP reduction gear before you use the BP, use the BP, then swap into your skill gear while the BP is readying.
2. Maximum range for Assault & BP:Rage is almost always 18 yalms. Some larger monsters you can do it from 20+ yalms. This can be a problem when some AOEs are 20 yalms. Being a SMN means constantly being aware of your position and that of your avatar.
3. As far as mandatory gear, I tell every SMN this: First thing you should buy is Sacrifice Torque. It's cheap and awesome for physical BPs like Predator Claws, and it can also wake you from sleep as long as you have an avatar out (because it damages you while equipped). Aside from that, empyrean & relic armor are the best thing to pursue. Relic armor has lots of BP timer reduction, empyrean has lots of skill & perp down. Yinyang Robe is also another great source for BP timer. Magian perpetuation staves are very nice as well, as is the Pet:MAB+12 staff from the ice path.
4. For Voidwatch specifically, you won't actually be doing much most of the time. Buffs & debuffs mainly. Earthen Armor is often good if you're in a tank party. Diabolos' Favor is good in mage party. Tidal Roar and/or Diamond Storm never hurts. Aside from that, I tend to just keep an avatar out (usually Diabolos for favor) and every 10 seconds I do Assault followed immediately by Retreat. This will often give a melee weakness hint. Between Assaults (10 sec recast), spam Dia for magic hints. SMN is a hint machine, without doing any significant damage. Of course once damage is on the menu, you can dish it out with BP:Rages.
____________________________Pergatory (Asura, Kupo LS) WHM99 ~ SMN99 ~ DRK99 "If you want to know me, look inside your heart." -Lao Tzu