Add a person, then link them to their parents and partners — one step at a time, whenever you have the details. This works the same whether you're adding a child, or an ancestor further back than anyone currently in the tree.
For a stillbirth or a baby who passed at birth, the same date can go in both fields — or leave the date blank if it isn't known and just add a note below.
A photo is a lovely thing to have, especially for anyone who's passed — completely optional, add one whenever you have it.
Both people need to already be added (step 1) before you can link them.