I do cook alot but rarely pies, partly I don't really like pastry that much but DH does and my toddler seems to as well. My first attempt flopped when I put a puff pastry top on a pie the bottom was undercooked and soggy while the top was getting burnt.
Maybe I should have cooled the filling first? Wrong oven temperature?
If that is all you want to do, it's actually very easy.
Cook all your pie filling, fill it into a ceramic or glass pie dish (not a metal one), and in the meanwhile, cut out a puff pastry shape that is a little bigger than the top of the dish you are using and bake it in the oven on a tray until it is completely cooked.
Once cooked, add it to the top of the pie dish that has the hot filling in it.
If you want to do pastry tops and not bottoms, you need a proper pie dish with a wide enough rim to be able to anchor the pastry to it.
Put your filling in the dish, then roll out the pastry. Firstly cut thin strips to create an edging to go on the rim and moisten rim slightly so that the pastry edging will stick to the rim. Then moisten the top of this edging and put the pastry lid over the pie, trimming and then crimping it (or whatever) round the edges to ensure that it is fairly leak proof.
You may need a ceramic or pyrex pastry support to go in the middle of the dish so that the pastry doesn't dip too much in the middle and you will also need a couple of air holes/slits so steam from the mixture can get out without lifting the pastry top off.