I've use a roller and a brush, although it can be sprayed - there should be package directions for spraying - basically use an airless sprayer.
I found that ultra-flat paint to be very forgiving, that is, easy to apply. It hides mistakes very well. I sprayed it on using an HVLP gravity feed sprayer from Lowes. I think the cost was about $30-$40. I mixed in a couple of ounces of Floetrol and it sprayed on beautifully. I did not thin the paint, other than the Floetrol. I recommend the Floetrol even if you brush or roll the paint, as it helps the paint level out and brush marks will disappear. If you have an air compressor, the HVLP sprayer is very simple to use and it does a great job.
Either way, proper prep is critical to getting a paint job that will last. Wash it down with a good degreaser and let dry. Use 80-grit sandpaper on any existing paint that you will be painting over. You want to just knock the top off and make sure that the old paint is clean, smooth, and reliably bonded to the surface. CARC dust is harmful. Use a respirator as you do not want to breath in the dust from sanding/grinding. Take care of any rust-through, surface rust, or greasy spots. Prime bare spots with a rust inhibiting primer and let dry thoroughly. I used Rustoleum rusty metal primer, and so far so good after a few months. Blow-off the surface with compressed air and wipe the surface down just prior to painting with denatured alcohol or mineral spirits. And then, paint. After it has cured for a week or two, wash it to remove any of the chalky residue that forms after this paint has been exposed to rain. I can't remember what causes it, but freshly cured latex paint will form a chalky appearance when it is first exposed to water. It generally only happens the first time it is exposed to water, and if you wash it off, it doesn't reoccur.
I'm not a pro and there may be better methods, but that has worked for me.
