Always take a pill as soon as you remember, and continue taking pills, 1 each day.
If you miss pills for 3 or more days in a row, or you start your next pack more than 3 days late, use condoms or avoid sex for the next 14 days. If you do have unprotected sex, consider using emergency contraception.
If those missed days are in the 3rd week of the pack, do as above AND skip the non-hormonal pills at the end of the pack (or the pill-free week), and start your new pack straight away instead.
If you miss any of the non-hormonal pills at the end of the pack, throw away the missed pills and keep taking the others as usual.
Also read the instructions you get in the box carefully should you miss a pill!