If you want to save time and a whole lot of work, you do what is called frilling the trees, You just take a hatchet and hit the tree at a downward angle, spray your round up or in my case I used 24D full strength in a hand held spray bottle and spray the cut area. Those trees will be dead in no time.
I saw it on a land managment program and did some research. For large trees you can do the same thing but it is recommended you cut all the way around the tree.

Or you could also drill a small hole in the trees and inject the round up into the holes. I prefer the cutting method, its quick and the herbicide of you choice last alot longer. Just remember to use a full concentrate to kill them for good.