Sometimes, a baby or a wearer or both need some time to get used to a carrier.
Start slowly. Pick a time when baby’s happy, and put baby in the carrier as quickly as possible. (If you’re using a ring sling, have it already adjusted so you need to make only minor adjustments once baby is in. If you’re using a wrap or other carrier, use a simple position.) Do not stand still while you putz with the carrier – move, bounce, walk, sway, sing, whatever your baby likes. Make sure baby’s comfortable and that you’re mimicking whatever in-arms position your baby enjoys. Wear the sling for 5-10 minutes or however long you’re both comfortable and happy. Do this several times a day, aiming to lengthen the amount of time slightly each time you wear the carrier.
Consider whether your baby just doesn’t like the particular position you’re trying. My oldest, though he loved being held laying down, detested the cradle position in the ring sling – he preferred to be sitting up. Many older babies don’t like the arms-in position and will resist any attempts to restrain their arms. Sometimes, older babies prefer to be worn on your back in a high back carry so they can look out at the world (but still have the ability to shut it all off if it’s too overwhelming).
If the problem is more that you need to get comfortable getting baby in and out with minimal crying, consider practicing first with a doll or stuffed animal, just to get the feel of the movements you’ll be using when handling your actual baby. When you’re practicing with your real baby, keep in mind that babies generally don’t have a high tolerance for the getting in/getting out process.