I assume this is "single" line reefing with 2 lines in the boom as the picture looks same as mine. It's earliest to do the line with the pulley out of the boom to be reinstalled fromthe aft end.
On the aft end of the pulley toward the stern: one end on the reef line ties to boom end cap (my cap has a hole that it threads thur and there the line is knoted) them thur the pulley back to the end cap, thur the roller on the end cap, up to the the reef eye in the sail and then back to the boom and is tied around it (I tie mine to the reed )eye.
The forward ties off to the boom or mast
, thur the boom to the pulley, back to the mast under the roller turning up, up thur the reef eye, back down to a roller at the bottom of the mast and then to the cockpit
or where ever the rfeefing is done from.
I found that on mast end it was easiler to take the rollers out of the slots to run the line and then reinstall. This sounds easlier than it was. When the sail is up the pulley in the boom should be toward the aft end so that it tightens toward forwad when you reef. When I had to do mine last year I think I just did a general internet
search on "single line reefing" till I found a diagram.