how the hell do they get it up there?

The half tops are placed flat on the deck on their respective sides abaft the mast. The gantline is sent down abaft the after cross-tree, and bent through the two most convenient futtock holes, in order that the half top may hang square whilst going aloft. An after gantline is bent to the after part, stopping it to the side. Lash two stout planks with spurs in them across the horns of the trestle-trees before and abaft the mast. “Sway away.”

Guy clear of the trestle-tress with the after gantline. When high enough, bear the half top round into its place, letting the midship part take against the spurs placed between the trestle-trees. Lower away and place, unbend the gantlines, send the sleepers aloft, and bolt all down. To remove the plank and spurs, the midship part must be wedged up.” – Commander G. S. Nares R.N.  – Seamanship (1868)

(This is just one of hundreds of detailed instructions in Commander Nares’ extraordinary handbook on Royal Navy procedures in a square-rigged ship. The tome, now very rare, also covers everything from provisioning to the stowage of anchors and cables. The breadth of practical knowledge required of ordinary sailors 150 years ago is astonishing.)