From Marshall Saddle, follow the signs continuing up the Aspen Trail to the north. After a couple tenths of a mile or so of moderate climbing, the trail begins to ascend sharply. The trail itself isn't terribly technical, just very steep for a good majority of it. The middle section of this trail in particular is the steepest, with more negotiable sections at the beginning and towards the end.
You'll pass through areas still ravaged by the Aspen Fire of 2003, although signs of new growth are popping up. Without as many trees, this upper section is a bit more exposed to the elements but you'll also find some great views. Even with the fire damage, the trail is still readily visible in most places and there are some cairns where necessary to help guide the way. Overall, it's a stiff test of climbing for a short trail but also a very fun and rewarding challenge.