Since the border punch has a specific width or distance from the edge of the paper where the cut occurs, you just need a reference point to create this exact distance. I have found from experience that if I trim off the edge of my paper to the exact point where one of the small holes in the punch design sits, then I can easily line up the punch when I turn the corner. I have an example in the picture on the left. The dark line represents the point where I will trim the edge of the paper. See how the line I have drawn just touches the small hole in the last scallop design.
Here is another picture where I have trimmed the paper. The paper I am using here in this example is two sided and the green side is the topside of the paper, the backside is a red and orange print. To turn our corner we'll need to work holding the punch in our hands upside down so we can see how the paper is lining up with our desired design.
Although it is a little hard to see in this picture, when I slide the new edge to be punched into the border punch I can use the small holes in the design to be sure that my corner is lined up. In the picture the first hole on the left lines up perfectly with the little hole that was created by the last scallop on the previous edge that I punched. All you need to do is punch the corner while holding the paper and punch upside down, then you can turn the punch back over and work as you normally would lining up the punched design with the design painted on the punch guide. Hope this helps those of you who are enjoying this Fiskars border paper punch!