Self-guided or guided?
For the big rim hike from Fira to Oia, you don't need a guide — the path hugs the cliff and is hard to lose. But in the villages, a good local guide turns a pretty walk into a real story: which house belonged to whom, why the churches are blue, where the cave homes came from. If you like context, book a guide for the towns and walk the rim yourself.
Whatever you choose, walk early. The midday sun is brutal and most of these routes have little shade. Wear proper shoes — the marble lanes are slippery and the rim path is loose stone in places. Carry water, a hat and a light layer for the wind, and you'll enjoy every one of these far more.