Him: starts on his back and kicks up his feet and legs so that his feet land just beyond his head. By this point, he should be balance on his shoulders, allowing me to step between his knees, facing one knee. I just need to drop my dick down into his hole after rimming him, edging him and getting a finger or two, (or 4 or more) down his hole to check for bends and kinks.
if the person is too inflexible, a sling makes a good alternate.