I did it a couple of years ago when I replaced the bearings and seals.
Like any greasable hub, fit on the gun the nipple on the cap and squeeze in grease till you just see the some grease coming out the back.
However if you want to do the job properly, remove the hub and "palm" the grease into each bearing so the grease fills the gaps between the rollers. It is also a good time to scrape out the old grease which may be decades old.
