I dry a lot of apples, however I have a dehydrator which makes the job easy. You can do it in a normal domestic oven provided you can set the temperature low enough, it needs to be around 50C so that it doesn't actually cook the apples, and a fan oven is much better.
Slice the apples to about 1cm thick - thicker than you think you need because they shrink a lot in drying and if they are too thin you end up with apple crisps. Of course if you like apple crisps, slice them thin! Dip the slices in salt water to help stop them from browning, lay them out on clean tea towels on shallow trays or on wire racks if you have them. Put them in the oven, prop the door slightly open and leave them until leathery. It takes many hours to get to that stage though.