Easy but can get messy, i used hammerite underseal (contains waxoyl) for places that i could get to with a brush and have direct contact.
For the rest i used the hammerite waxoyl but the high pressure tin with pump then can spray it into places that dont have direct contact with the road (places like wheel arches are considered to have direct contact). Other places light above the fuel tank and above exhaust are unlikely to see direct contact with stones and water so be ok with just waxoyl.
Much easier if you can get the car on a proper lift, i do mine on axle stands and lying on my back in the road
Worth noting you should remove any heavy rust and the parts being sealed should be dry. You will likely have light surface rust in places, wirebrush them to sort them anything heavier then that, hammerite rust removing gel does a good job.
I was lucky with mine it hadn't been sealed since coming into the country but was was fairly clean when i bought it, just gave it the underside and archs a wash and allowed to dry before sealing.