Im no expert, and I dont fully understand legatos, but I can do them no problem.
I think you take 3 notes, and then you pick the first, and go through the 3 notes to make 6 notes in 1 bar by hammer-ons and pull-offs.
example: on the G string only. 7>9<6>7>9<7. The arrows indicating > hammer-on and < pull-off, picking only the first note.
Learn speed picking, haste, shredding, sweep picking, and slash's stuff will make more sense to you.