Certainly we need to stress that a parent or other attached caregiver would be the best "lovie" a child could have. There is no substitute for the warm, loving arms of a caregiver and the security that they provide for the child. However, we realize that sometimes a lovie (such as a stuffed animal or blanket) can be an appropriate tool, and as long as it is not overused, it can be comforting to some children. Some high-needs children require almost constant contact with a parent or caregiver. Sometimes this level of contact is not possible, especially in a household with multiple children. For instance, if you need to lay the baby down to take a nap, but the baby wants you to lie with him or her and you are not able to, a lovie might be an acceptable fill-in. If the lovie carries the scent of the primary caregiver (usually mom), it can be that much more soothing to the child. Additionally, for a child who is in a daycare, a lovie can be a comfort from home.
Introducing a lovie to a young infant could be as simple as tucking it into the sling with her while you carry her, or tucking it in with her as she sleeps contentedly in bed (with or without you). This should set up the lovie/sleep association. For an older toddler, introducing a lovie could be a bit more challenging since he will be more resistant to the caregiver substitute. Showing interest in it yourself may be enough to spark some curiosity for your child. Some children might enjoy being surprised with one, while others may prefer going to pick one out.
The most important thing to keep in mind is that the lovie should be associated with positivity to the child. Putting a child in a room to cry it out with a lovie sets up a negative association and is unfair to the child. Try to be understanding in the process of introducing a lovie, and realize that it may take time and gentle persistence for your child to accept one.