Currently, we vote for one representative per riding. The issue with that is that (hypothetically) one riding could have a million people and another could have 100 people. But both of them would have the same amount of power in Parliament because 1 riding = 1 mp.
How would that work in a proportional election system? Is there one candidate per X number of citizens in an area? Wouldn't cities be over represented? Wouldn't there be one candidate to cover very large sparsely inhabited areas that might not have the same needs from one spot to another?
A pure proportional voting would have no ridding/districts, I think this is more common for municipal and provincial level elections.
If the countries are divided into independent-ish states or provinces, they can divide the population by the number of available seats - that means a province with 10% of the population, ideally would have close to 10% of the seats.
I am not a big fan of breaking down provinces further because the more you divide into smaller districts, more votes are thrown away, and you open it up for gerrymandering.
Canada seems to do a good job dividing seats per province, the problem is that the provinces break it down into districts and use first past the post voting to elect officials.