I'm unfamiliar with Geneva voting procedures, but the way I understand the description it works the same as federal parliamentary votes, which is what I describe here.
Seats are distributed among the various lists according to their share of all votes, so a party that gets 23% of all votes gets 23% of all available seats. A vote for a candidate automatically also give a vote to the list he's proposed on, while empty spots on a list that names a party give a vote to that party only, not to any one candidate. The party ranks its candidates according to the number of votes each candidate got individually, more votes gets higher priority. The party distributes the seats until all have been used up, in case of surplus seats (in your example the 75% get 15 seats) it will look for additional "candidates" (usually within its own ranks) after the vote to fill them with.
Usually, at least with the big parties, "list" is synonymous with "party", a candidate vote automatically gives also a vote to his party.
However, multiple parties can group up on one single list, in this case every candidate vote counts for that list/group. Computing the number of seats for those parties requires an additional step, but for the voting itself you can pretend for that shared list to make one single (temporary) party and safely ignore the details - this is also what I do in this post, except in this paragraph: I don't differentiate between "party" and "list that contains more than one parties".
As for filling out the list/form:
You can pick any list, including the empty one, and adjust to your liking, provided you follow these rules:
- everything must be written by hand
- no obscenities, insults, propaganda etc
- a list must contain at least one candidate name
- you may strike thru preprinted candidates, and either leave the spot blank or replace with any other candidate
- any and all candidate names can be used to fill an empty list place, whatever their party, e.g. you can place LCG candidates on the LPR list
- any one candidate can be voted for up to two times. If you vote for him/her thrice or more those surplus votes will be ignored, these list places will be considered empty (see below)
- empty list spots give no candidate vote, but they do give a list vote provided the list name, or a list number, has been filled in the list head
- list name and number may be left blank, in which case you forfeit the party votes the empty spots (if any) give automatically. You may strike thru the list name and number of any preprinted list, and either leave it blank or fill one in
A list has a list number and a name. If you use the emtpy list and want to provide a list name it's useful to fill out the list number as well, similar applies to the candidates and their numbers (assuming they do have one).
When in doubt you can wait filling out part or the whole stuff and ask the people in the voting office, there will be at least two persons present and everybody will just love(!) to answer your questions, especially if they come from a foreigner