Hi mate, whoa what a job youve gone and made for yourself.
I hope I can help you.
First you need to clean the wall down with a stirr brush to remove any loose bits and dust. Then check out all the pointing, if in bad condition repoint where needed.
Then when you have made up your mortar (1/4 or 1/5 mix) with a couple of caps of plasterciser damp down the wall or if walls are to dusty or dirty apply a pva solution(1/4).
Apply your first coat(scratch) over the area you are going to do first and try to keep as level as possible,(it dosnt have to be smooth). Then move on to another area and repeat the process.
When you have covered one complete wall leave for an hour or so and then lightly scratch with a scratch comb or float with a few nails in the end.
Leave to thouroughly dry out, at least a week or two.
When applying the second coat do just as with the first coat with the same mortar mix. when finished leave for an hour or so again before fining downThis is basically rubbing the float over the rendering to obtain a flat finish. When doing this high spots will rub down and low spots will show up, you can then just flick some mortar into the low spots and fine down.
When this is fully done you can lightly sponge the whole area down with a good stiff sponge. This brings the cement to the surface( you will see sand falling off the wall as you do this) and gives a good professional finished look. PS forgot to tell you before putting on the scratch coat check the level of the walls with a straight edge. If walls are to out of plumb you will have to fill in the low spots(dubbing out) befeore starting.
OH yeah and each coat should be around 10mm thick but dont panik to much if its not...!
Hope this helps you out.