Fuji - Hakone pass reservations and use

hi , we are looking to purchase Fuji - Hakone passes for our trip. I'm aware that you buy the passes from Shinjuku station, and that you need to reserve seats on the highway bus to Kawaguchiko - but how do you reserve? Do we need to do this beforehand, before we get there and buy the pass?
Also, does anyone know what these transport routes around the lakes/ Hakone etc are like on weekends? we'd be there on a Saturday and Sunday. Is the tranport limited then?
Thanks!!

I'm aware that you buy the passes from Shinjuku station, and that you need to reserve seats on the highway bus to Kawaguchiko

No, you don't have to. You could also travel the loop in the opposite direction. But when taking the bus between Shinjuku and Kawaguchiko you indeed have to make advance reservations. You can do so by phone, internet or at the bus terminal (see details here: http://bus-en.fujikyu.co.jp/highway/detail/id/1/). I am not sure whether the ticket counter where you buy the pass may also be able to make your reservation.

Also, does anyone know what these transport routes around the lakes/ Hakone etc are like on weekends? we'd be there on a Saturday and Sunday. Is the tranport limited then?

It is busier on weekends. More people, more crowded buses. Bus timetables are mostly the same on weekdays and weekends.

When you are buying the pass in Odakyu counter at Shinjuku station, the easiest way is to make the staff call the bus company and reserve the specific bus for you, they will usually do it anyway.