for concave area’s you normally apply the vinyl until the stress in the vinyl will cause wrinkels in the next stroke, then you heat the entire piece until the stress dissapeares, you the recomence until there is again to much stress, heat again, and repeat the process,
if it’s a wrap it’s easier, just applie until the concave area, heat the remaining piece of vinyl and with both hands pull the vinyl over the concave area while spreading your arms apart so you opening the vinyl while stretching it, much faster, much easier, just practice and you’ll get it