I begin at the head and work my way down to the tail and hind legs. Since the clippers may get hot after a time, you will want to stop periodically and let them cool down. If you wish to give your dogs specialty cuts like a poodle or a cocker cut, you should follow the directions that come with the clippers. You can also get books that have diagrams on giving special cuts. The clippers come with attachments that cut hair to the desired length for whatever effect you are trying to achieve.
Toes are a particularly vexing problem.
Dogs do not like you messing with their feet, and especially dislike the vibrations from the clipers on their feet. I suggest trimming with scissors and then using toenail clippers on the very long toenails. You can use the clippers on the feet but the dogs will put up a fight. Care should be exercised with the toe nail clippers as if you cut the nails too short you will nick the vein that runs part way down the nail. You can purchase special nail clippers that seal off the vein and keep you from creating a mess if you cut the nails too short.