What’s your problem with it?
You require an extra hero recruitment card for a given hero, to activate their Starnet. Hence, you first have the hero, then you get an extra recruitment card for them. If they’re in the tavern, this is easy: you simply use as much essence as they require to recruit, then recruit them for a second time. After you have an extra recruitment card for a hero that you already have, you can go to ‘Starnet’ in the Hero window and click on the central sphere (‘Starball’) to activate Starnet.
Heroes can have Starnet updated multiple times, by a quantity depending upon their stars (Heavy cavalryman is 1-star, Frost Predator is 4-star, etc.) Seemingly, 1 and 2 star heroes have 1 ability to unlock, 3 and 4 stars have 3 abilities, and 5 stars have 5. I’m not sure about 6 stars, but by this pattern they would also have 5. Still, I’m not entirely sure if this is an exhaustive limit. Each upgrade gives them a slight stat boost, plus a special ability. Each upgrade requires a further recruitment card for that hero. The abilities given for each level of Starnet typically have the following sequence: first two involve a buff that targets a certain other type of unit (eg. Fighter, Mage, Tank), third gives a percentage increase to a certain group of stats, fourth involves an increase in things like DMGG Rate or Reduce DMG Rate (less conventional stats, often concerning rates), and fifth is a more miscellaneous ability that might enhance the hero’s power or your team.
It’s slightly similar to how you evolve relics, by getting multiple copies of that relic.
Once you’ve activated Starnet, you can increase the hero’s stats by clicking on the spheres representing them, and this costs a certain amount of essence. Increasing the spheres by one level only takes 15 essence each, that essence requirement soon increases to higher amounts.
For about 1000 diamonds (or possibly some ambers instead), you can use ‘reborn’ on your hero’s Starnet, returning to you all of the resources used on them. This includes the essence, as well as the recruitment cards (that you can recycle for the essence used on them.) This might sound costly, but if you don’t use it that much, it’s a moderate cost.