If you TOOK MY ADVICE and parked along Mapleton, walk west on the sidewalk past a few parking spots on the street. You'll come to this break in a wooden fence. Your hike begins here!
You'll know you're in the right place by this sign. It is a bit misleading; it only mentions the Sanitas Valley Trail, but this is a general access point for any of the Sanitas trails you should choose. Boulder's Open Space and Mountain Parks (OSMP) team is extremely diligent about their signage. This sign communicates that dogs are allowed on this trail segment on-leash, bikes are not allowed, and horses are allowed (although I've never seen one here).
Walk though a short wooded section along a ditch. This is perhaps the narrowest part of your entire route. Accordingly, watch out for trail runners and people talking on phones, as they rarely yield, especially when they are near the end of their journey.
In a few hundred feet you will come to a large trail junction. Take the wide trail to the right of this sign. There is a ton of stuff here, and it totally makes sense for you to be overwhelmed by it. You will find: a map of the area's trails, color-coded by dog leash regulations; announcements and other regulations; a sign displaying the dog regulations in the immediate area; temporary signage; free, compostable dog-poop bags; a garbage can for ONLY dog poop in said compostable bags; and a trash can for all other trash, including dog poop in non-compostable bags. Overwhelming? Yes. Helpful? Also yes.
The Sanitas Valley trail does not change much as you gently ascend. This photo gives you the gist of the first mile of your route. Follow the wide gravel trail up the sunny valley floor.
At a few points along the Valley Trail, you will have opportunities to branch off to the right and ascend Dakota Ridge, a shadier ridge walk that also takes you up the East Ridge Trail. Stay left at these junctions to remain on your route.
Here is the view back down Sanitas Valley from its high point. Your trail is joined by the Dakota Ridge Trail as you continue to ascend.
Stay left here, as the very steep Goat Trail descends to the right into the neighborhoods below.
Soon, there is another trail junction at which you will stay left. The trail to the right also provides neighborhood access. There is a convenient bear-proof trash can to the right, just a bit off the trail. That trash can is a welcome sight when carrying dog poop.
The trail begins to switchback up Mount Sanitas' east ridge. The steepness increases markedly, but you will the trail remains wide for the time being.
The trail gets rougher as you make your way up the steep east ridge. Stairs like this are interspersed with flatter trail segments as well as small scrambles over rocks.
Behold the crux of Mount Sanitas' east ridge. It looks steeper in a photo than it really is. Stick to the crack on the left, pointing just to the right of the left-most tree. You can see a trail marker sticking up. Just look around for rocks that can serve as steps.
Near the top of this little rock formation. A trail marker indicates to make a left here. Do that; do not pop through the gap in the rocks straight ahead!
You made it! There is a metal pole and a rock with a plaque to commemorate the summit. On the map, the endpoint is a bit off - that marker represents the Mount Sanitas Trail-Lion's Lair Trail junction. Enjoy the views, and chat with the occasional OSMP ranger as well as some of Boulder's fittest people. Plenty of ultra-runners regularly do laps on Sanitas.