You are not “appealing a decision” as such. Because a decision has not been made against your child personally, but your child has been refused a place because based on the schools admission criteria other children have been allocated places above your child because they are siblings, LAC, have special needs, live closer or whatever the criteria are.
An appeal however would generally be based on the fact that you think the preferred school would be a better fit for “your” child .This could be because it offers a particular subject like a specific language, because your child is musical and it has amazing musical facilities, because your child has a particular skill like maths or sport that the preferred school offers. The place would be offered if the panel felt that on balance the inclusion of your child would be of more value to your child than of inconvenience to the school.
So if your child has been offered a place, but not at your first preference school, your first reaction should be to accept that place because a) it makes no difference to an appeal or a place on a waiting list and b) your child will not be left in no place limbo which can be very unsettling.
Then go on the waiting list for the preferred school when the waiting lists open because you never know, and in some places there is a lot of churn.
Then look at what the preferred school offers that you can argue, and show, will benefit your child in an appeal.
Schools and local authorities publish the dates for appeal deadlines nearer the time.