If this is the first time you've greased them you will have to fill the cavity between the front and rear bearings with grease. Otherwise you will not get any grease to the front bearing as that cavity has to be full of grease. Use a manual grease gun, take your time, spin the wheels, and watch for grease at the front. It can take a couple tubes to do all four wheels. Then clean the excess grease out so it doesn't make a mess.
Be advised that some people say they have followed the directions from Dexter and have had seal failures. Some people say they have never had any issues. Ya pays yer money and ya takes yer cherce.