If your baby is going to be bottle-fed when you aren't there, start introducing the bottle before you go back to work. Just remember that it's best to breastfeed a baby exclusively for the first 3 to 4 weeks. This lets your body establish milk production before separation from the baby. It also helps to prevent "nipple confusion" between the breast and the artificial nipple. Introducing the bottle for substitute or supplementary feedings before 4 weeks can possibly lead to a decrease in milk production.
How you introduce the bottle when getting ready to return to work may depend on the length of your maternity leave. If you must return to work 4 to 8 weeks after giving birth, start by adding a bottle of your expressed milk about 1 or 2 weeks beforehand. Choose a feeding when you may normally be away from your baby and only give them a bottle during that time. This may help your baby adapt more easily to the change, as by then it will be a part of their regular routine. You may want to limit the amount in the bottle, then your baby will more likely be ready to breastfeed in 2 to 3 hours.
For a longer maternity leave, you may want to introduce a bottle with your milk by 4 to 6 weeks. Keep offering small amounts of your milk from a bottle on most days. This will keep the routine a familiar part of your baby's life. It can also help make things easier when you go back to work.
Some babies won't take a bottle from their mother. Your partner or someone else may need to give them the bottle. This will also help your baby adjust to having someone other than you give the feedings. Also, not all bottle nipples are alike. Some tend to be more compatible with breastfeeding than others, and some babies prefer 1 type over another. Talk with your baby's healthcare provider or a lactation consultant for advice if you are having any problems introducing a bottle. Be aware that some babies may have such a strong preference for the breast that they will only take just enough from a bottle to "get by" while their mother is away. They will make up for this by nursing more when you are around.
If you don't have to return to work until the second part of your baby's first year, you may not even need to introduce a bottle. Older babies often do well when drinking directly from a cup or a sipper-type cup.