This is some info I post to another thread,
Bear in mind it was a newly boarded wall.
So in you case, I would repair major damage with a basecoat. I would tape over any cracks or minor holes, prime the walls with a dilute PVA 4-1, let first coat dry then apply a second coat wait until it goes tacky then start applying your first coat.
This is the method I use when plastering a boarded wall.
Joints taped up with nylon scrim.
*one bag(25kg) of multi covers 10sqm
*one bag of multi to one standard bucket of water.
*water in mixing bucket first, then pour plaster in until powered is just heaping over top of water, then mix up, add more plaster a bit at a time until bag is empty, all mixed up through with no lumps.
*slightly wet down ligger board, hawk and trowel
*mix on to ligger
*then spread the plaster on wall starting at top left to right (right handed) in motions of three, leaving a thickness of about 2mm. Then up from middle to top again motions of three, then bottom to middle motions of three.
*once area is covered, flatten and fill
*tools cleaned and wet down
*mix second coat 1/2 (25kg) bag for 10sqm
*half bag to just over half a bucket of water, this mix wants to be a little wetter.
*mix up as with first coat
*spread again starting at top left to right (right hander) 1mm thickness, motions of three.
*you can then again go middle to top and bottom to middle, motions of three. or you can go bottom to top.
*once second coat has covered area, flatten and fill.
*clean tools
*wait for it to take
*cup of tea and garibaldi/custard cream or hob nob if it's Fridays and chill
*once taken, wet trowel wall, taking out any imperfections.
*clean tools
*you can dry trowel and polish if you wish, but if the surface is going to be painted, I find the paint takes better after the wet troweling.
for tiled areas I would leave the finish coat slightly rougher, this would allow a good key for the tiles.