Back to the Horsetooth Mountain Ride map.
I'm going to level with you right away here: I am afraid of this trail. It has some very nice rideable sections, in fact all of it is rideable, but some of the sections are life-threatening. Every time I've tried to really ride (not weanie-walk) this trail, I have had a close encounter with our planet of the painful kind ("gee: who knew dirt could be so hard!").
The way to ride this trail is to climb the radio towers service road to it's intersection with the Mill Creek Trail. At this point, you are almost at the top of the service road, the radio towers are only another half mile or so. Climbing the Mill Creek Trail with a bicylcle is an exercise in carrying one's bike. It's just too danged steep to climb on wheels without assistance from an internal combustion engine. So, assuming that you are still willing to try this, climb the service road.
The Mill Creek Trail starts out innocently enough, and in fact I think that's what scares me about it. At first blush, it seems like a pretty reasonable trail. It is in fact an old Jeep road. I doubt a Jeep has run on it in 30 years. There is one section near the top that's about quarter of a mile long, probably an 11% grade covered with loose rocks ranging in size from fist to melon. Once you get into a descent on this slope, it's probably safer to take your chances staying on the bike than trying to stop. It's sort of like riding a bicycles down a slope covered with ball bearings of various sizes.
After that, I characterize the Mill Creek experience as a series of gotchas. You'll come around a corner, see a section that looks techy but do-able, then realize once you've committed to ride it that there are a bunch of roots or rocks that you hadn't seen at first that are going to kill you. Trying to stop at that point is the most dangerous thing you can do, but riding it is pretty dangerous too.
Where a helmet for this ride (and full body armour if you have it).