Always use quickdraws on sport climbing anchors to rig top-ropes. The lowering anchors you find on sport routes are almost always two bolts, usually with steel rings or quick links for threading your rope through.
After climbing a sport route, you may want to set up a top-rope to do another lap on it or for your friends to climb it. The best and safest way to rig a top-rope from sport anchors is to clip a couple quickdraws to the bolts and run your rope through the bottom carabiners.
When you get to the anchors after leading the route, clip a quickdraw to each bolt and then clip your rope through the bottom carabiners. Make sure the carabiners have their gates opposed to each other so they don’t accidently open. It’s safest to use at least one quickdraw with a locking carabiner for the rope to run through. If both bottom carabiners are auto-locking, you’ll have lots of redundancy as well as plenty of safety. Also check the carabiners to make sure they don’t have lots of wear from previous top-rope ascents. The rope sliding through carabiners quickly wears them out.
The lazy climber runs his rope directly through the bolt hangers or rings, which greatly increases wear on them and decreases the life of the hardware. Remember that someone is going to have to eventually replace that hardware, usually at a great cost. Always use your own gear like quickdraws or carabiners and slings to rig top-ropes on sport climbs and be a responsible climber.