If you have access to a boat, try fishing flats east of Gotts Point in 10-20 feet of water. The north shore east of Gotts Point can be good in spring from April-June as catfish move to shallow waters to spawn. The west basin is also a good area. Check the flats along the south shore of the west basin (Caldwell Dam side) in 8-15' deep. Remember, boating is only upen April - September, so check with the Deer Flat Refuge office for details.
If you don't have a boat, access is a bit more challenging. Try fishing the access area around Gotts Point (the end of Greenhurst Road), and the shoreline along the Kingfisher Trail to the east of Gotts Point. There is also access near the Deer Flat Refuge headquarters. The long shoreline point near the headquarters and along the Nampa Dam are worth trying. Otherwise, the Caldwell Dam has a lot of access, but I'm not sure how good the catfishing can be.
The most important thing to keep in mind is using good bait. Freshly cut fish like carp, suckers, pikeminnows or perch is the best bet. If you can't get fresh cut bait, try the more traditional catfish baits available at your local bait shop.