i took an old camshaft and cut the bearing journals off it --if your engine has different size bearings for each cam bearing bore you have to modify each journal for each bearing shell .
I bored/drilled a 3/4 inch hole thru the center -- made a backup washer just slightly larger than the journal to butt up against the cam bearing shell -- got a length of 3/4 inch cold rolled steel and drilled and tapped a 3/8 hole for a retainer washer and bolt and made a shoulder on the 3/4 inch bar back far enough to butt up against the cam bearing journal and to stack the backup washer and journal onto the shaft and install the retaining washer and bolt.
.install the cam bearing shell onto the journal you are going to install and drive the cam bearing into the cam bearing bore making sure you are centered and square with the bore.
i used this homemade tool on all my engines and never had a problem.