Slugs are added to boltroped sails by adding brass grommets just aft of the boltrope. The grommets are added using a die and anvil into which you set the spur grommet then hit the die/anvil with a hammer (of course you cut a small hole into sailcloth to preliminarily locate the grommet. The grommets are mounted just aft of the boltrope.
To connect the grommets you can either use plastic bails (which either snap-on or are held together with a small screw) or (better) handsew the connection the the slug with some small webbing and waxed heavy sailmakers twine, using a sailmakers needle.
If you have a heavy duty sewing machine available you dont even need grommets as you can simply attach the slugs to the sail with several layers of small webbing and attach that just behind the boltrope with a "box - X" set of stitching. Grommets will give the 'best looking' job.
Go to www.sailrite.com
and look at their online catalogue. If you have any diffuculty in size selection just give Jeff or Jim Grant or one of the others at Sailrite to help you select the correct materials, etc. They also have a 'how - to' or 'instruction' section on their website: http://sailrite.com/Tips/Installation%20of%20Slugs.htm
The grommet die will be the most expensive component.
Such modifications are fully explained in the book "Sailmakers Apprentice" .... although adding grommets & slugs at a boltrope is such an easy job, you shouldnt need the book just to do this.
hope this helps.