Rain gutters should slope 1/2 inch for every 10 feet towards a downspout.
If gutter is longer than 40', they should be pitched from the center to downspouts on each end.
Let me make this easy for you. Go to Home-ideas.org on your search bar and type in a search for installing downspouts. You will get step by step easy instructions for installation of downspouts. Make sure you know the square foot area of your roof as it will make a difference in the size of gutter you will be using. If you live in a rainy climate this is important.
You also need to pay attention to where the water is going when it is running down the spout. To much water in the wrong place to often can lead to damage to your home at the foundation in that particular area.
Directing water in the right spot will also reduce erosion. I like to catch the water and use it for gardening purposes since it is quite handy and it save on the water bill. If you have any more questions, consult your cities local building codes dept. for answers to how they would like to see this job done.
By the way, snap your line according to the mounting screwholes on each bracket not the gutter itself. When you are finished screwing in the brackets, you will find that the gutters will snap into place according to the pitch of the brackets.