It depends, if you take trains that don’t need a reservation than you only need your Interrail Pass
(Paper: write on it the trains you take that day, and write on the ticket the day and the month of that day. Mobile: choose your connection on the railplaner app)
From Lyon to Liege you could take a TGV (Reservation compulsory) from Lyon to Brussel and there change on to IC train (no reservation needed) or TGV to Paris and change there for Thallys to Liege or look at the railplaner app if there are some trains in france (TER) without reservation.
→ I’m not sure, but I read that french people get a free reservation for the TGV to leave with Interrail France and get back. I would ask at a SNCF desk. If that is so, I would take the TGV to Brussel and change there.
Ok I hope that is the case. I thank you for your very precise detailed. I will take the necessary steps with the SNCF. Because paying a reservation in addition to the pass is expensive…