Both Sierra and Whiskey are pretty easy to find - S at J10 M4 and W near J11
If you go for W, look for Majeski Stadium, and you will see the lakes to the east of Reading, the the railway line points to the field.
If you join at S, keep a watch for traffic incoming from W over your left shoulder. Also, check the circuit patterns, for 21 and 25, you keep inside the M4 downwind, and turn base inside the dual carriageway that runs N/S from
M4 J8/9. I have seen plenty going out over Maidenhead, you are then outside the circuit and in the LHR airspace.