Consider the room temperature and how many layers you need. Generally, you'll want to dress your baby in the same amount of layers that you'd have for sleep (or maybe one additional layer if you feel it's needed). 

Don't over-bundle. Overheating can cause an increased risk for sudden infant death syndrome (SIDS), so you'll want to look out for signs that your baby's too warm: sweating, flushed skin, or the baby's chest feeling hot to the touch. Your infant may be comfortable in only a onesie or long-sleeved pajamas under the sleep sack, depending on the ambient temperature and thickness of the wearable blanket.