Less than 24 hours late
If you are less than 24 hours late taking your pill, take your late pill as soon as you remember. Then continue taking the rest of your pills at your usual time. It's okay if this means you'll take 2 pills on the same day - this is safe to do though you may have some nausea. You do not need to use backup contraception.
Missed 1 pill
If you are 24 hours late taking your pill, but less than 48 hours late (you missed just 1 pill), take your missed pill as soon as you remember. Then continue taking the rest of your pills at your usual time. This means you'll take 2 pills on the same day, which is okay. You may want to use backup contraception until you've been back on the pill for 7 days just to play it safe.
Missed 2 pills
If you missed two pills in a row, you should take two pills today and two pills tomorrow. Then continue taking the rest of your pills normally - one pill per day at your usual time. If the 2 pills you missed were in the final week of active pills, skip the hormone-free week by finishing the active pills in the current pack and starting a new pack the very next day. Whenever you miss two pills in a row, you should also use condoms or avoid sex until you have been back on the pill for 7 straight days.
Missed more than 2 pills
If you miss more than two pills, you essentially are re-starting your birth control from scratch. If you have a new pack on hand, great! You will start with this fresh pack. If you do not, you should order one. Your insurance may not pay for a new pack though. You can either call your insurance for an override or purchase a pack out of pocket. You will take one tablet daily from the new pack starting the day you get them, or the day you realized you missed more than 2 pills. You will do this at whatever time you have as your set time, and continue with one daily for the rest of the pack. You will need to use backup contraception until you have been back on your birth control for 7 days.