T5 spa is very busy. Assuming you are not in F, go straight there after frisking and try to get a spot. If you are out of luck, ask if there is availability at the spa at B gate. It is often quieter over there. If you are on a B or C flight, all will be simple. If you are out of A it is far more complex to go to B, though instructions are available on the BA Board. For me, the latter would be too stressful.