Drop the lower unit you will see where the shift rod goes into the lower, screw the shift rod out and remove the 6 bolts holding the cover down. Where the rod goes through the cover there is a bushing and an Oring, the Oring usually is the culprit, use a fine pick to dislodge it clean the shaft and replace the Oring. Use some oil and gently screw the rod down through the Oring, place the gasket extend the rod so you can easily screw it back into the shift converter. Once the rod is screwed back in place slide the cover down, use a little non hardening gasket compound on both sides of the gasket. I save the compound for last makes for less mess.
Phil
No matter what your beliefs are "GOD BLESS AMERICA"
Fully retired marine tech near 60 years in the biz.