Get a competing job offer. Whatever they offer you, ask for a comparable raise of your current workplace. Be very explicit about it being non-negotiable. Whether to mention the competing job offer is a matter of taste; it helps prove you mean business, but it can damage trust. Either way: if they say no you leave.
This only works if, and because, you are ready to walk away. It's the easiest form of negotiating. But it does require you to actually be able to walk away!
This strategy works extremely well if you are actually underpaid, and if you are you might not need that offer at all: I once bluffed my way into a 16% raise that was issued on the spot.
This is not a success story - it only worked because I was an incredibly poor negotiator and signed on for at least 20% under my market value.
This only works if, and because, you are ready to walk away. It's the easiest form of negotiating. But it does require you to actually be able to walk away!