The westbound Amtrak leaves Denver at 7:15 AM.How can I get from Fort Collins to there?
Arrow / Black Hills Stages have an early bus, but it's unclear if it only stops in Greeley or also in Ft. Collins. Greyhound lists this bus as departing from Ft. Collins (5:45am) but Arrow / Black Hills lists it only from Greeley. You may want to call them to confirm.
This bus is scheduled to arrive Denver Greyhound station at 6:40am. It's a 12 minute walk from there to Denver Union Station. So this is cutting it close, consider the risk of having to stay an extra day in Denver and take the next train out, Amtrak tickets are often flexible, check the exact conditions.
In case this bus does, in fact, only stop in Greeley (5:40am) then you could try to get a ride/cab from Ft. Collins to catch it there.
If you're going on a weekday (Monday through Friday and not a holiday), you could take the Bustang North Line commuter bus service. See my answer on Getting from Denver Amtrak to Fort Collins for details. For instance, there is a bus leaving downtown Fort Collins at 5:40 AM and arriving at Union Station at 7:05 AM. You can also catch it at I-25 and Harmony Road.
(Note that as of this writing, the westbound Amtrak California Zephyr arrives in Denver at 7:15 AM, but doesn't depart until 8:05 AM. So this doesn't cut it as close as it appears. If you prefer more or less extra time, there are other buses scheduled 30 minutes earlier and later.)