i use waxoyl with my compressor and sprayer, but its too late to do it now, its a summer job. if you do it now and get damp or salt trapped under the sealant you will only accelerate the corrosion.
if you must do it now you need to clean the arches and underbody with sugar soap, then get it perfectly dry (at least 24hrs in warm garage to dry), then spray it somewhere warm and dry (garage), and leave it for another 24 hours before driving. also remeber to mask off the rest of the car because accidents happen, and undersealants are hard to remove from paintwork and trim, trust me i've made this mistake more than once and it wont happen again.
also remember that any existing corrosion needs to be treated before you apply the underseal, if you dont do this it will still rot. easiest way to treat rust patches is to get a wire brush bit for a drill to reomve the excess corrsion, then apply some rust treatment gel such as jenoseel. this needs to be done some time before the underseal is applied, and only once the car is dry, but its an important stage so its worth the hassle and time.