Determine the best option for your trip and vehicle. There are options whether you don't have a roof rack, or have a partial or full rack system. For those without roof racks, a hitch mount is a viable option and crossbars are available to add to partial racks that only have the two parallel side rooftop rails. A luggage rack or hauling store often installs vehicle hitches, if necessary.
Decide between using a rooftop carrier such as a cargo bag versus a roof rack depending on the cargo. Rooftop carrier options include cargo bag in both soft or hard shell options. Although both provide protection from the weather, soft shells are usually less expensive. Whereas a roof rack or basket doesn't protect from the elements, but it does put fewer limits on your cargo capacity and allows you to transport odd-shaped such as fishing poles.
luggage rack system manufacturer's user manual for proper loading instructions and to know the rack's weight limit; exceeding the weight limit causes damage to the vehicle. Also, check that the luggage rack system is securely fastened to the vehicle prior to loading up for the trip.
Load the luggage onto the roof starting with the heaviest items. If you have rooftop carriers then load the luggage or other cargo into the bags. For extra protection, place items that can be damaged from rain in plastic before loading. If you have more than one cargo bag or luggage on a rack or in a basket, place zippers facing toward the center of the roof, and use a lock on the zippers.
Secure the luggage using luggage straps, cargo bag straps or a bungee net. For roof racks or baskets use straps or a bungee net to keep items in place.