When planning the placement of the buttonhole for my almost finished garment, I follow these steps so my buttonhole is successful. I try the garment on and get a basic idea of where I want it to be. I then carefully measure the area keeping everything parallel and even. I mark the buttonhole with thread because I don't care for marking pencils. Thread can always be removed and will blend in with the fabric. It is great to follow the pattern's instructions and marking for buttonhole placement. Just keep in mind that those markings should be used as guidelines and customizing your garment to be your very own is what making your own clothing is all about.
I then machine stitch the buttonhole using the style I like best from the built in stitches my machine offers. After checking to see if I am happy with the result, I carefully cut the fabric in between with a seam ripper. There is usually some fabric and interfacing visible after doing this. I trim away what I can but then I prefer to add an extra step. I handstitch the buttonhole on top of the machine stitches using a buttonhole stitch.