It’s good that you’re tackling this issue before first communion day. Make sure your child understands that you expect reverent behavior; do not encourage or tolerate any kind of silliness around the act of receiving communion, even when you are practicing.
Children have a natural curiosity about new things, especially something as special and mysterious as the Eucharist. Give your child an unconsecrated host and let her hold it in her hands, look at it, and take her time tasting it and consuming it. She won’t be able to do this when she actually receives communion, but it will allow her to satisfy her curiosity. Then show her how you receive and ask her to imitate you. Some people, finding the dry host difficult to swallow all at once, let it dissolve slowly in the mouth. Encourage your child to try this and to say a special prayer to Jesus while she is consuming the host.
If she will receive the Eucharist under both species, the moisture of the precious Blood can make swallowing the host easier. Have her practice receiving from the cup with watered-down wine or grape juice so that she knows what to expect. Above all, when first communion day comes, relax. If you have prepared her well, trust that the solemnity of the liturgy will carry her and that God will give her the grace to receive Christ’s Body eagerly and with dignity.