A common way to approach this is to walk through the flight and write down each frequency you'll need in order. Your route is a bit unclear (are you departing Aspen or landing there?) but let's say it's KBDU-KASE-KDEN and it's a VFR flight. And I'm assuming you're only asking about comms, not nav.
You think through what you're doing at each point and then write down the station, frequency and what you plan to say, request or listen for, as the case may be. For example, you're departing KBDU; it's uncontrolled so you'll check the AWOS and then use the CTAF to announce your intentions as you taxi and depart.
After that the route to KASE seems to be through uncontrolled airspace but you'll probably want to get the Empire and Copper Mtn AWOS along the way. And if you want flight following then you'll have to call Denver Center to get it.
KASE is controlled, so you'll need to get their ATIS, then call the tower, and after landing you'll be handed off to Ground. Departing KASE, you'll get the ATIS, call Ground, then the Tower and after departure maybe Denver Center again for flight following.
And so on...
You'll end up with a frequency log like this:
- KBDU - AWOS - 118.825 - Get altimeter, wind, etc.
- KBDU - CTAF - 122.725 - Announce taxi and departure
- Denver Ctr - 126.1 - Request flight following
- Empire AWOS - 134.325 - Get WX and compare to forecast; change flight plan if needed
- KASE - ATIS - 120.4 - Get WX and runway in use
- KASE - Tower - 118.85 - Announce landing intentions (check two way comms before entering class D!)
- KASE - Ground -121.9 - Request taxi to FBO
This can be a really useful planning exercise, especially in unfamiliar areas. If you're unsure of the phraseology or procedures for any item on the list then you can talk to your instructor about it, and listen to real comms on LiveATC. KDEN in particular would (will?) be busy, it's class B.
But remember that plans change. ATC may give you an unexpected frequency, or you may copy down a frequency incorrectly and then find no one is listening. Make sure that you update your log in flight as needed; don't fixate on it being 'correct' to the point where you ignore instructions or information to the contrary; and be able to look up frequencies quickly on your charts or flight software.