The easiest way to get from Toulouse Airport to Cahors by car is to follow the signs to Bordeaux (A62 North). If in doubt, stay in the right lane of the Toulouse ring road, as there are some late-posted turns. The A62 proper starts with a toll booth (gare de péage), where you take a ticket. Stay on the A62 until the Montauban exit. After paying the toll (about 3 euros), follow the A20 (by now occasionally posted to Limoges and Paris). Take the Montauban bypass to exit 60, posted Cahors via RN 20. That's the old main road via Caussade to Cahors. It's a good road and a bit more interesting than the autoroute.
At the South end of Cahors there are three roundabouts in a row.
The first is by a railway viaduct: keep on the main road.
At the second, take the right fork to Cahors (Centre Ville).
Past another roundabout and a couple of traffic lights,
and you'll come to a stone bridge that goes over the Lot into Cahors.
At the North end of the bridge is a roundabout:
right to Cabessut, straight ahead to central Cahors, left to the
railway station.