Watercress and cucumber sandwiches are a staple of afternoon tea. I know you prepare the ...?

Just cream cheese [no need for butter] on the bread, I slice the cucumber thinly and lay it on kitchen roll, before adding it to the sandwiches.
The watercress should be dried the same way after washing, I chop it through a little, so it does'nt dangle out of the sandwich, I put watercress in sandwiches with sliced boiled egg / egg salad / mayonnaise, with just a light sprinkle of salt on the watercress.
If using egg salad or egg mayo, you only need to butter the watercress side / slice.

Don't put anything but cream cheese on the sandwiches, though mostly people just use butter. You don't need to marinate the cucumbers -- it will make the thin slices of bread very soggy --, and you don't need do anything to the watercress beyond washing it. You could soak it in water with a little white vinegar in it for half an hour, rinse it thoroughly and make sure it's fully dry before you use it.

no, I do NOT marinate the cucumbers..
the way we made them was buttered bread, fresh cucumber slices, a sprinkle of salt and chopped fresh dill. top with more buttered bread. Done.