I'm assuming by jackstay you meant jackyard, as in those spars the tops'ls are attached to?
If so, they are set from on deck. The haliard is attached via a hitch, like a timber hitch, the out-haul or sheet runs trough a block out on the gaff and belays, usually, at the boom jaws. There's a tack line for the bottom corner of the tops'l that makes off at the fife-rail, a deck ring, or the boom jaws. There's sometimes two tack lines so the foot can be brought over the peak halliard on the other tack; the sail is typically set to windward of the mains'l, so it's downdraft doesn't back-wind the main.
On the other hand, a jackstay is a rod, line, or cable that the sails and some other items are bent (attached) to. Typicially there's one, or even two on top of the yards, and some times up the back of a mast where the spanker or driver slides up and down on rings instead of on the mast itself, or a spencer mast, with hoops or lacing. There were sometimes jackstays mounted on booms and gaffs to bend the sail too, but I don't think that was very common.