The older the child gets, the more absorbency needed in the diaper. I found that adding more layers to my pre-fold diaper made my diapers take forever to dry, so I chose to make diaper liners instead. This is great because you can add or subtract the amount of liners you use for when you know the baby will require more (Like at night or getting older).
I have made these out of both flannel and t-shirts. I no longer make diapers out of flannel because it is rather stiff, but flannel works well for diaper liners. If I have enough t-shirts, I actually prefer to make liners out of them; they dry softer on the clothesline and don't fray.
First cut four rectangles. 5"x12" or 4"x13" is a good size.
Stack them. I first sew a straight seam around the edges to make sure all the layers are connected and won't bunch up in the wash.
Then I cut around the edge to make it all nice and even.
Then if you have a serger, that would be great, but I zigzag around the edges to sew in all the edges and keep them together.
And there you have a nice soft diaper liner.