In the Send Mail step you can specify the "To:" with a calculation or a direct field reference: in your case, use the latter choice to select "field y".
To get your mail body right, you should study how the Calculation Engine works, and how one can combine ("concatenate") field and variable references with static ("literal") text; a simple example:
"Dear " & User:.nameFirst &
"please find enclosed the draft for our new marketing campaign:" &
You could also calculate this body text someplace else (a field, a variable in the script) in the same way, then reference it here.