It's hard to give definitive guidelines on water ratios, depends on how dry or damp the sand already is. It takes a little experience to judge the when the mix is just right; for the first mix go easy on water, you can always add more if required. A plasticiser would be a good idea too- washing up liquid is not the same as a proper plasticiser, except for very small jobs. As for other dos and donts:
Don't overwork or over trowel the render- this brings fines to the surface, creates a "laitence" and your render may craze.
Use a good sharp sand for plastering work- soft sands have a lot of fines and are for brick/ blocklaying only.
Always work from a wet edge back, left to right- this preserves the life of your steel trowel. I presume you'll be using a plastic float to rub up the surface on completion. Keep the background damp to reduce suction and prevent drying out- especially with the current weather.
