A useful tip to remember is to make the folded insert approx. 0.5cm smaller both on length and width. This means that the insert will sit neatly and won't show. Also, I usually stick it to the front so that when the card is opened, the insert opens at the same time. However, if the basic cardstock is a bit lightweight and I have added several layers to the front, then I stick the insert to the back page along its whole length, as this helps to strengthen it.
HTH
Judy C