Basically just, read them as you get them, and then you'll have some inkling of them in your mind when you come to do assignments and think "hang on I think I read something that I can use here...". Definitely don't wait until your assignment to do the readings, as you'll just end up running out of time, particularly as you may well be given a bibliogrpahy or suggested readings for a particular essay title anyway which you would need to read at the time. Trying to do that with the week on week readings would be near impossible, I think.
The readings are not "extra" work generally - they are the main source of learning for the most part. Your lectures will normally just be an introduction and/or overview of some topic, or focus on a particular special case study or similar. The readings are what you need to be doing to actually learn what you need for the module for the most part, so you should work on them as they are posted/released. You then will leverage them in your essays, assignments, and in seminars/tutorials based on having already read them and synthesised that knowledge.