[Taqwus]

Prereqs:
Mine tech, which you mention you have. 5 levels max.

Explosive warheads tech, to put something in the mines. 3 levels max. You might as well go for 3 first, because it's a fairly cheap area and it's strangely hard to kill an organic or crystalline ship w/ 100-pt warheads.

Select the 'Design' menu, then 'Unit Designs', then create a new design, selecting Mine. Choose your favorite mine size, fill it with warheads. The VehicleSize.txt file, if memory serves, hints that at least at one point intentions were to include other components, which may shift the balance towards large mines; for now, I prefer small, since sweepers sweep a certain fixed number of mines regardless of mine size.

Then you'll want a minelayer -- design a ship w/ one minelaying component and a number of cargo bays (i.e. transport hulls are good for this). Cloaking is also good, 'natch. One minelaying component will lay unlimited mines per turn outside of combat, anyway.