You can get reservations for the Eurostar here
https://www.b-europe.com/EN/Booking/Pass#TravelWish
Paris-Geneva via the Eurail reservations service. But see my next reply for a different option.
Switzerland - Italy, and Italy domestic trains here. Add your pass where it says 1x adult, and then search for 'one way tickets and day tickets'
https://shop.oebbtickets.at/en/ticket
Rome to Prague, it depends which route you take, but you'll need reservations within Italy, and crossing from Italy to Switzerland or Austria, as above on ÖBB. No reservations needed within Switzerland or Austria.
No reservations needed Prague-Berlin (but highly recommended). Same with domestic trains within Germany, and Germany-Brussels. You can get these on the ÖBB website (search for seat reservations), or the DB website.
From Berlin, youcan also take the sleeper to Brussels, to connect with the Eurostar. You buy the reservation from
https://www.europeansleeper.eu/en