We've been out a couple of times this year and came back empty handed. Still kind of early for mackerel. Speak to any old sea dog in your local harbour and you'll hear conflicting stories and advice about what to catch.
We were fishing out of Rosehearty at the weekend with ragworm (check out the film Dune with Sting) and you'll get the general idea! If you don't fancy digging it up at low tide, then any decent tackle shop will sell you it, or peeler crab, squid, etc.
Mackerel is great when they're in decent condition, but cod, pollack, wrasse and dab are more fun. Bait up (plenty youtube videos), then using 6-8oz weight sink the hook right to the bottom. Don't troll, don't even play with it... just wait. Normally we throw our lines then crack open the coffee and sandwiches!
Rocky ground and wrecks offer the best results.