Use the largest Herring and fillet them and clean them well. You can either keep the skin on the fillets after de scaling or remove the skin. I remove it. To make it easier to remove I soak the fillets in a brine over night (12 hrs is best). For that brine use enough water to soak all the Herring you will be using and add a couple tbsp of white sugar and 1/3 cup of pickling salt. Get the brine hot enough to dissolve all the sugar and salt. A light simmer for awhile should dissolve it all. Then let the brine cool in fridge very well until adding the Herring to it. After the fillets soak in that brine for 12 hrs I then I rinse them very well and the skin peels off pretty easily this way. It also makes the herring a little bit tougher so that they roll easier without falling apart.
Use your favorite kind of mustard and spread a little bit on each inside part of the fillet you are going to roll up. Dijon goes well with Herring but you can use any kind. Also put whatever ingredients you want on the fillet before rolling. Some people like a mix of little pieces of pickled veggies but you can use fresh veggies too as the pickling marinade you make up will pickle those fresh veggies over time too but if you plan on eating the rollmops after only a short period of time in the jars then using pickled veggies is better. I use our home made pickles and onion to put in each roll mop. Then roll them up and put one or two toothpicks through each one to hold the roll together.
For the pickling marinade I put all my pickling herbs/spices in boiling water, cider vinegar and sugar. You can use white wine vinegar too or whatever vinegar you want but make sure its a concentrated enough to qualify as a pickling brine. Bring to a boil then reduce to light simmer and simmer for about an hour. I then strain out the solids. Some people leave all those ingredients in the pickling jars. Doing the latter will increase the flavor of the pickling over time. Myself I like it to be not a super strong herb/spice pickling and more of a cider vinegar with a touch of sweet pickling flavor so I strain that stuff out after boiling. Let that pickling marinade liquid get very cold in the fridge before pickling the Herring. If it's even the slightest bit warm it will cook the Herring and the meat will fall apart. I use about a 2 to 1 ratio of cider vinegar to water but that depends on how you like your pickling liquid. You can use less vinegar but never less than a 1 to 1 ratio or it won't pickle. For my pickling liquid my ratio of sugar to liquid is: 1 liter of water/vinegar mixture I add 1/4 cup of white sugar and my spices/herbs. But you need to make enough to cover how ever many rollmops you are making so that amount of pickling liquid will be different for everyone depending on how big or small of a batch you are making.
For the spices/herbs I put in my pickling marinade I use: whole peppercorns, whole all spice, whole juniper berries, whole cloves, mustard seeds all slightly broken in a mortal pestle so flavors are released, and a couple of bay leafs.
Put your rollmops in pickling jars then pour in cold pickling liquid in to cover them all. Make sure they are all covered at least an inch or more over top. Flavor/texture is best at least a couple weeks of pickling but they will be decent after a week pickling. They last a very long time pickled. We still have a couple jars left from last winter and they are great.