See, there's two things that make Cas so confident, even if he's never had or given a massage in his entire life. One: Cas knows the construction of Dean's body down to the last muscle fiber, the last cell and atom. He knows how to pull and push a muscle to make it malleable, how they should feel in his hands when they're new, relaxed, healthy. How they should feel attached to the bone.

Two: Castiel sees all humans as works of art, but Dean's body he worships as both his own work and God's. God may have created Dean first, but Cas restored him, and Dean's body is beautiful because of the sureness of Castiel's hands. He's confident.

Whether or not that confidence is well-founded, he'll see.

He kneels on the bed beside Dean. They don't have lotions or oils but it's not like Cas would have thought to use anything like that anyway, but luckily the room is hot and Dean is just a little bit sweaty so Cas's hands don't scrape over his skin with friction.

Shoulders, upper arms and upper back first. Then the neck, down to the lower back, sides, back to the shoulders and all over again. That he's touching Dean in a way that could be perceived as sexual doesn't even occur to Cas, he's so absorbed in doing this correctly.