Unless you bolt on something to attach luggage to, there's no real way to do it. Like most cruisers tt's simply not designed to take luggage in it's base form. That's an optional extra for those who want it, otherwise it's a "naked bike" with all that implies.
If you want to maintain it's sharp looks when there's no luggage on it, try easy brackets which have a pair of small discrete pegs on either side of the rear fender. You can also go for K-Drive bags that have a small mounting bracket that the bags can lock onto. Brent at
Newspeedmaster sells both.
Triumph also sell their luggage rack without the backrest as a cheaper option.
