How do they work
You place them on a gas giant that has fuel. They wont work anyplace else. There they refill the tank of your specific sorium miner ship by the amount that is indicated on its design screen annually.
What is a good design
Depends what you want them for. I see at least 3 possible design routes - all of which I did in different games.
1. Fuel collector base: Basically nothing but harvester and fuel storage modules + very little engines (or even none, so you tractor them in place). I put on as much storage so that one years harvest fits in usually (+ a little extra, because commanders give bonuses), but that is entirely open to personal judgement. You shouldn't make the storage too small, because then you will have to frequent message interrupts that somewhere your fuel tanks are over 90% full, but you shouldn't do too much either, because it simply takes away from the harvester module space, for which it is actually there.
I always have at least tiny engines on these now, even when I intend to tractor them, because that will enable them to fly to the closest moon of a gas giant (if he has one at all that is), and deposit their fuel there. It is a comfort function, because then you can use the conditional orders ("deposit fuel at nearest colony when over 90% full"), and collect fuel from there after your own schedule.
2. Fuel collector ship: If you do not mind reduced efficiency, or a good gas giant is acceptably close to one of your colonies, then you might consider to not bother with tankers who move between collection site and colonies at all, and simply let the collectors themselves do the flying. I think it is not too useful, but at least in one game I came across a situation where this was the best option.(-> a 300m+ sorium Jupiter aligned in a way so that the nearest colony was Earth, leading to fully automated and near limitless fuel generation for me)
3. Independent ship: A ship that actually has other purpose like freighting, geo survey, or passenger transport, which can gain infinite distance capability in theory by having some percentage of the hull being sorium harvesters. All you need to do when the fuel is low, is find a giant somewhere (doesn't even need to be a rich one), and refill. That refilling will take a lot of time though if the tank was depleted a lot (..years usually), and is not very efficient to do this if you have colonies or actual tankers in at least moderate range. It is a nice concept for a deep space vehicle though if you do not intent to ever turn for example.
...Wont really work with military designs without many further tweaks, because those of course consume MSP on top of the fuel.
What to do when they are built
You can organize the fuel mining in two general ways. Either you are sending tankers with the "refill from target fleet" command towards your harvesters when they are full, and then go "unload 90% fuel on colony" (do this before as well if the tankers happen to be full). ..Or you use the default orders as described above, which require at least some engines on the harvesters. If there are moons around the target giant, then go right-click on the closest of them and do "create colony", so they will drop any fuel there. Then, if you see some need in one of your worlds, just fly a couple tankers over and collect from the storage.
Both these ways can be automated, so that tankers fly their collection route in regular intervals.(either with the "cycle orders" checkbox in the task force window, or semi manually via saved order templates on the right there, which I usually do)
