start with a bare wall, loose material removed, brushed down and lightly wetted .
apply plaster around the edge like a frame ; i got the left hand one flat then some horizontal ribs to project the levels to the other side, then complete the right screed
fill in the blank areas bring level with the frame. This is only the first coat but it is brought level, if it isn’t what is the point? After trowelling I went over it with a couple of straight edges; one long, one short – up and down and side to side sawing movement across the surface just to take off the high spots.
key with a comb ready for the next coat, which will be easy as this is already more or less level, some shrinkage will cause this first coat to be less level than it was at the start, this wont crack because there is enough hair in it to stop that but it will still shrink.
So there you have it, a very small bit of wall I know but the principles are the same all the time, get the plaster level with the first coat not the second, the second coat just brings the plaster to the correct depth. If your first coat is not level what was it’s purpose?