Mincior made a tutorial on DIY double sided PCBs using vinyl stickers as your toner transfer substrate. We wrote about his vinyl sticker method in an earlier post, but this one covers synchronization between the two layers as well.
This is a different way of making a double sided PCB, having one feature in common with the previously presented option – it also relies on vinyl sticker. For this reason we’re going to call it double sided vinyl sticker PCB. As you will soon discover, I have used pins for synchronizing the two sides. It’s a little bit more complicated BUT it should provide more accuracy with aligning the two sides.