|I used to spend time marking buttonholes and then sewing carefully. Either my pins fell out or I couldn't see the marks because the presserfoot would cover them.
I do not have an automatic buttonholer, I have a 5 step, ie sew one side, turn a knob, sew the end, turn a knob etc. So, the length of each buttonhole has to be marked.
Now, I mark where the buttonholes should go and the length of each (still with pins), but about an inch to the side of where it actually should go. I then line up the garment edge with a mark I've made on the machine arm as I would do with edgestitching. This means that every buttonhole will be exactly 1/2" from the edge for example if that's what the pattern calls for, as long as I keep the garment edge lined up with the mark. With the garment lined up on the edge, I then look to the side and line up with the first pin, start the buttonhole, watch for the second pin for length, finish the buttonhole.
If I had an automatic buttonholer, I'd set it up, then line the garment front up along the edge, find the first pin mark for the placement and press GO (or so I've heard how those automatic things work).
Don't cut the threads between buttonholes. Sew the first one, pull the garment clear with some thread tails, line up for the second one, continue until all are done, then remove from the machine and clip all the front and back buttonhole threads at once.
Sew the buttons on by machine. It's another tip written up here and it is well worth it. Use a glue stick on the back of each button to stick it on in the right place, and again, don't cut the threads inbetween until you're done them all.