There are lots of good birding places in Anza Borrego – it’s a big place. You should plan to do most of your birding very early in the morning when the birds are most active; as the day heats up it gets harder to find birds. Exact locations to bird really depend on your target species, but one of the most popular sites would be the settling ponds at the Water Treatment Plant on Borrego Springs Road. The ponds are sometimes dry and then the bird life drops significantly.
We’ve had good luck seeing Bighorn Sheep at the water source right by the parking lot for the Anza Borrego State Park campground (there is a fee to enter–if it is even open now). White-tailed Antelope Squirrels are also at that location in large numbers. Plants are going to be hard in late August because most will not have reproductive parts to help with ID.
Due to the pandemic, San Diego Audubon Society’s field trips are all suspended, but when they resume, coming along on those field trips, which are free, will introduce you to good places to go birding. There are a handful of sites in the Borrego Springs area typically visited on the field trips. There are also places like Agua Caliente County Park, Scissor’s Crossing, and others that are good for birding. You can look at the eBird hotspots map to see where they are.