The pistons do not require greasing. Simply check they are able to move in and out freely and clean off any dust.

Clean the pad carriers so that the new pads can slide freely and before you fit the pads lubricate all metal to metal contact points with a good quality high temperature grease designed for the job. Normally I use Coppaslip but some on here will disagree. When I fitted new pads earlier this year there was a tube of lubricant in the box. Thought I had better use it and guess what it was, Coppaslip. Sometimes its necessary to coat the back of the pads with Coppaslip to prevent noise but it does not help keeping them free moving.

When you change the pads, simply retract the pistons all the way back intothe caliper. When doing so make sure the bleed nipple is open so you do not pump fluid backwards through the ABS.
It should not take any excessive effort to retract the pistons and they should move smoothly.

As Skid says, clean up the calipers especially the contact points for the new pads. I too use Copperslip.

Also dismantle the slide pins and use brake cleaner to remove old traces of lube. Re-lube with copperslip (it only needs a little). Assemble the carrier without pads and ensure it moves freely on its pins, then fit the pads.

If you have never done so, bleed fresh fluid through each corner (all four). If this is not something you are practiced at, it should only cost about £60 at an independant.

If you find you were experiencing uneven pad wear this could be down to a sticking piston, sticking guide pins, sticking pads (in the carrier) or even excess water/oxygen content in the fluid at one corner (makes the fluid too compressible absorbing some of the transmitted energy).

Just to add:

check the condition of the seals aroiund each piston and give them a thorough clean with brake cleaner but do not disturb them from their seats (I find a soft toothbush useful, but be gentle). Any signs of a split or dirt and/or water ingress to the piston then consider having them rebuilt (or simply go for an exchange set of recon units).