Guides are not compulsory, though don't ask a guide that question! As the tallest volcano in Indonesia, and the tallest non-Papuan peak, Kerinci is very popular with Indonesian hikers. Very few of them take guides.
I had to ask a local or two the way to the trailhead the first time I climbed. It's easy, in that there's an obvious motorable track all the way to the trailhead, but there are a few such tracks on the mountain slope, and there's no sign anywhere pointing to the trailhead. You'll have to ask. The trailhead's about 5km from the tiger statue in Kersik Tua.
The trail is obvious to above the treeline. From there to the top, it's very rocky, and at times slippery scree, and there's no real trail as such. It's easy to find your way up: just keep climbing. It's much more difficult to find the correct way back down, if there's a lot of cloud around. You have to be careful on the way up to note your route, so that you can retrace your steps back down. Once you hit the trees, it's easy again. It's slippery all the way down though, it's not a mountain you can run down.
I started climbing (from Kersik Tua) at 6am when I summited in one day. I reached the trailhead about 6:50, reached the peak about 11:20, had about 25 minutes at the top, in cloudy conditions, and was back in Kersik Tua by 6 in the evening. You'll save yourself a bit of time if you take an ojek to the trailhead, but you'll still want an early start.