Placing the Seam
Before you do any actual cutting, you should consider where to place the seam on the window. You will usually have options when deciding: do you want it vertical or horizontal? Higher or lower? It may depend on the width of the roll you are working with, but generally you'll want to place it where it is least noticable. If you have tall windows, placing it at the top will make it harder to see. If it is darker at the bottom or people tend to look toward the top more often, then placing it at the bottom will be the best method.
Cutting the Seam
The second factor to consider when doing this technique properly is the cutting. Some customers try to cut the two pieces as true as possible with a straight-edge, then line them up and install. Or, they make sure to use the factory edges of the film when ''butting'' the two ends together. This can work well, but most times it is difficult to cut the film perfectly straight, or using the factory edges creates too much waste.
The best method to splicing is to overlap the two pieces about 1/2'' where they meet. Install the entirety of the pieces, then use a sharp blade to cut down the center of the overlap. Since the pieces are installed and adhere firm on the window, they shouldn't move or peel while you are slicing. Remove the sliced strips of film and you are left with a tight seam.
--Nathan, DIY Window Tint Professional