You don't put 'spackle' on at all. After taping the drywall seams you 'mud' them with drywall compound. -It is a totally different substance from 'spackle', which should be used only to fill small holes and irregularities in wall.
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It is not advisable to put drywall mud over wallpaper backer. The reason is, the moisture in the drywall mud may cause the backing to loosen and this will result in your entire wall coating coming away.