It's been a while, but I'll try to remember the gist of it...
Mosquitoes at Diamond Lake and on the trail were ravenous, so bring your bug juice. Follow the Mt. Thielsen Trail until it crosses the PCT and peters out. After that, you start heading up towards the summit horn. As you make your way up, you need to stay to the left. There is a big gully/valley off to the left and you want to stay fairly close to the edge. If you get too far to the right, it really prolongs the scree slog.
The summit horn itself is about a hundred feet or less of fun class 3 (with maybe a little class 4) scrambling. You don't need a rope unless you aren't comfortable down-climbing and would rather rap.
I think there are one or two technical routes on the other side, but I can't give you any beta about those.