You can buy all this stuff ready made - just go to Ikea and ignore the furniture - but that’s not quite the point. Here are some useful elements (the amounts are right for about 8 people):
Jane Grigson is very good on the pickling of herring in her wonderful Fish Book. The Danish style takes a bit of doing – you need to pickle your fish (and it’s a slow pickle, over a week or two), then soak it, then marinate it in a sweet and sour dressing. I happen to prefer them straight up, no marinade.
1 tsp alspice berries
1 tsp coriander seeds
2 tsp peppercorns
70g caster sugar
4 mackerel fillets
This is the gravad laks method applied to mackerel. It’s a quick(ish) pickle – three days.
First make the cure: coarsely grind the juniper berries, coriander seeds and peppercorns in a mortar; mix with the salt and sugar.
Lay a quarter of the cure out on a plate, then two of the mackerel fillets, skin side down, then half the cure, then the other two fillets, skin side up, then the rest of the cure.
Cover with cling film and leave in a cool place for three days – could be the fridge, but it seems odd to use a fridge when pickling, considering it’s an alternative to refrigeration.
To serve, wash the cure off the fish, and pat dry with kitchen paper, and slice in half lengthways, taking the opportunity to remove any lingering bones.