Being on a really tight budget leaves no room for error. I write my meal plan then write my list by category - so I can grab everything I need from each department. Then I price up every item using the Woolworths website, rounding up to the nearest $0.50, so I can add it all up and see where I sit with my budget. If I'm slightly over I'll adjust some things, but more often than not I'm under, and I can incorporate items from my running list of staples, like paper towel, soy sauce, toothpaste etc, as I list these well before they're needed so they're always purchased before we run out.
Once I'm done and it matches my budget, I'll write one or two extra items from the running list at the bottom, in case we come in under still and I can grab something extra.