Shenhe

5★

CryoPolearmSupport

TL;DR

Burst, tap Skill, swap out. The Skill hands every teammate a few charges that make their next Cryo hits land harder. The Burst drops a field that softens enemies to Cryo. All of it is Cryo only, so she goes in a team with someone whose damage is Cryo.

Rotation

Burst (Q) to put the field down. It sits there for 12 seconds.
Tap Skill (E). Everyone in the party picks up the charges.
Swap to your Cryo damage dealer and spend them.
Pop back and tap Skill (E) again when it's off cooldown.

Abilities

Skill (E)	Tapped, it gives the whole party 5 charges lasting 10 seconds, on a 10 second cooldown. Held, it's 7 charges lasting 15 seconds, on a 15 second cooldown. Each charge makes one instance of Cryo damage hit harder, by an amount based on Shenhe's Attack.
Burst (Q)	80 Energy, back in 20 seconds. A field lasting 12 seconds. It lowers enemy Cryo and Physical resistance, ticks away at them and gives whoever is standing in it 15% Cryo damage.

The one mechanic to understand

Tap and hold buff different buttons.

Past the charge count, the two versions of the Skill hand out different bonuses. Tapping gives the party 15% more Skill and Burst damage for 10 seconds. Holding gives 15% more normal, charged and plunging attack damage for 15 seconds. So hold if your Cryo character does their damage by swinging or shooting. Tap if it comes out of their own Skill and Burst. Tapping is the more common answer, partly because the shorter cooldown lets you get two of them into a rotation.

What actually matters

The two or three real decisions. Everything else is noise.

Attack is her buff stat. The charges scale off Shenhe's Attack, so Attack goes in every slot that isn't paying for Energy. Her own damage is small enough to ignore.
She needs a lot of Energy Recharge. 80 Energy is one of the most expensive Bursts in the game and the field is the bigger half of what she does. Around 180% if she's the only other Cryo character, less with a second one making particles.
A crowd eats the charges. One attack hitting four enemies spends four charges, not one, so a wide Cryo hit can clear the whole lot in a single swing. It's the same buff either way, it just runs out sooner than you'd expect against groups.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge, then Attack. Favonius Lance is a standard banner 4-star and its particles go a long way towards paying for the Burst, though it wants roughly 30% Crit Rate to fire reliably. The Catch is free from the fishing shop if you'd rather not spend stats on Crit Rate.
Artifacts	4-piece Noblesse Oblige. 20% team Attack on Burst, which stacks with everything else she's handing out.
Main stats	Attack or Energy Recharge sands, Attack goblet, Attack circlet. A Cryo damage goblet is wasted here, since it only boosts damage she isn't dealing.

Works well with

Cryo

Hydro

Cryo is the requirement rather than a suggestion: nothing she does touches damage of any other element. Hydro on top of that freezes things, which is where most Cryo teams end up.

Don't bother

Crit DMG. She swings often enough to notice it in a fight and never often enough for it to matter.
Levelling her Normal Attacks. Skill first, then Burst.
Running her with a Pyro or Electro carry. The charges will sit there unspent.
Staying on the field after the Skill. The charges are already handed out and the field runs without her.

Want the full theorycrafting version? Read Shenhe on KeqingMains.