Forgive my ignorance but how exactly do you stack vouchers?

I’ve heard this mentioned a bit in the run up to 11.11 and was wondering exactly what this means in practice?

For example I assume that you firstly collect the AliExpress 11.11 vouchers? And then go to the coins page and try to buy through there to get coin discounts and then apply the AliExpress voucher? But how do store vouchers come into it? Do you just need to try to collect those too?