Traditional Plinth Blocks have straight edges.  Due to the curved sides at the top of the base blocks,  it's position will effect the placement and reveal of the case molding. That reveal will be about 3/8".

The top of the Base block will be flush with the door jamb.   Adhere the Plinth blocks with construction adhesive, and pin or clamp in place.  Discurage nailing in place. Any nailing will need a pre drilled hole and filled afterwords.  The area indicated with the arrow may need some fill behind the door jamb.  Bondo works well prior to installing the blocks or calk as needed. Any skilled finish carpenter will have no problems with installation.

Position the base blocks and adhere.  Hold the case blocks in position and draw a line to determine  measurement of case molding needed. Cut case molding and install. Adhere case blocks in position.   Repeat for other side.  Measure distance between Case blocks and cut and install top case molding.  Untill the adheseive harders , you can make slight adjustmens to the base and case blocks position.

Should the blocks need any repairs, use FGRC,(provided with purchase)water, and a little white glue mixture.  Only mix what you can use in about 10 minutes, as it will harden and can not be solidified again.