We took a DIY day trip to Assisi from Rome back in 2019. Took the train from Termini, as someone else suggested. Once we got to Assisi, we got on a local bus from the train station to the hilltop old town (I think we bought the bus tickets in a news stand in the Assisi train station, a couple Euros each). It was not difficult to find. We rode the bus to the very last stop, as suggested in our RS Italy guidebook. Then we used the RS App with our phones to listen to Rick's walking audio tour (we find them informative and easy to use). Since we took the bus all of the way to the last stop, it was a nice downhill walk, and we started/stopped the tour as needed to check other things out/go into the various churches. After spending a few hours, we caught the bus back to the station, and also visited the cathedral near the train station (I think it's the one that they built around St Francis' original chapel). All in all, it was a great day. We enjoyed seeing the old town and all of the St Francis sites. Definitely possible without an organized tour.