Most babies will usually need to burp half way through a feed, as well as at the end of a feed. Burping your baby twice can also 'wake' them a little to finish the feed, if they have fallen asleep while feeding. (Some babies will want to feed again quite soon, if they have only had 'half' a feed).
To burp your baby you can gently sit them up, and forward on your knee. Some parents will place their baby over their shoulder to burp them. Babies will usually bring up a little milk with the burp. This is normal. So if you don't want to end up with regurgitated formula on your leg (or down your back) you can place a cloth nappy, hand towel or bib under your baby's chin (or on your shoulder) to help absorb this. As you sit them up, make sure you support their head, by holding your hand under their chin (without being tight around their neck).
Image 11-36 shows a baby in the sitting position (asleep) with her head supported for burping.
It is fairly common for bottle fed babies to gulp down some air while feeding. To minimise this you should make sure that the bottle is held at an angle while feeding, making sure the milk fills the teat throughout the feed. Burping will expel any air swallowed, but not all babies need to burp. Past generations tended to place great emphasis on 'burping or winding' a baby, this has unfortunately lead to new parents to worry unnecessarily if their baby does not burp. If nothing has happened after 5 minutes or so, then it probably won't. There is no need to 'force' a burp. You can gently rub your baby's back if you like, but there is no need to pat them forcefully.