The best time to run conduit is before your pour the concrete slab. This allows you to install the conduit under the slab in a location where it is well protected. PVC conduit is typically used for these applications because it is weather-tight and doesn't corrode due to exposure to the acidic concrete. Installing conduit one size larger than you need is wise to accommodate future expansion of the electrical system.
Things You'll Need
Chalk a line from the start of the conduit run to the end by holding the chalk line tight between the two points and snapping it with your fingers.
Remove 3-5 inches of gravel along the chalk line with a shovel. Remove any large rocks and keep the base of the trench level.
Lay the PVC conduit in the trench from the start of the run to the end.
Fill the trench back in with the material removed. Rake the ground as flat as possible.
Tamp the ground along the conduit with a hand tamper. Be careful not tamp so hard that it breaks the conduit.
Never use a mechanical tamper over conduit. The strength of this style of tamper will break the pipe.