If you have any doubt on the freshness/quality of your salmon, freeze it first then defrost it before making the gravlax to get rid of any bacteria. If it has been previously frozen or is sushi grade, no need to do this.
Trim any thin pieces from your salmon. They will likely over-cure and are best used in some other way fresh. Carefully check to see if there are any bones and remove. Cut the salmon in two equal-sized pieces. This can be down the central spine if a wide fillet so you have in effect two squares.
Remove the feathery leaves from the dill stems and finely chop them.
Mix together the cure of salt, sugar, pepper, dill and lemon zest in a small bowl.
Place one of the pieces of salmon, skin side down, in a freezer bag inside a dish just larger than the piece of salmon. Spoon most of the cure over the top and spread evenly.
Place the other piece of salmon on top, skin side up. It should be the opposite way round from the bottom piece if possible (ie if bottom piece is thick on the left, have the other piece thick side to the left. Spoon over the last of the cure and rub that plus any cure that has escaped out the sides on the sides and over the top of the salmon.
Close up the freezer bag and weigh down the top - for example put a heavy dish on top or a plate weighed down with something heavy (such as some cans). Place in the fridge and leave it to cure, turning every 12-24 hours, for 2-3 days until the salmon goes firm and slightly opaque.
Rub off the excess cure (some stray dill is fine) and pat the fish dry. Slice the salmon thinly across the grain, removing any skin, and serve.