Discounts didn't stack, but amounts did (e.g. if you have 2x Gold Tickets, your guaranteed allocation is 10x Eggs at 5% discount). To use the Ticket, simply have it on your wallet during the mint. The checkout UI will offer you to use the Ticket if it detects that you have it. WL NFT Tickets can guarantee allocation only during the WL sale period, after which the sale will be conducted on first-come, first-serve basis.