One food at a time
First, you should only introduce one food at a time in order to check for reactions. So give them the new food and wait at least 3-4 days before trying anything else new.
If you're worried about them not liking it, remember that it may just be the newness that freaks them out. Smile when you give it to them and cheer them on so they know that new doesn't mean bad. If they keep spitting it out or turning away then stop and try something else for a while. You could also mix it with something they like to tune down the flavor.
We always did the new food at least 3 days in a row and Avery usually liked it by the third day. Actually, I can't think of a food she doesn't like!