If the Crankshaft is Not Installed

Apply new engine oil to the piston, inside the ring compressor, and the cylinder bore. Then attach the ring compressor to the piston.
Remove the connecting rod caps, then check that the bearing is securely in place.
Position the arrow (A) facing the cam chain side of the engine.

Fig 1: Identifying Arrow Position On Piston Facing Cam Chain Side Of Engine
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Position the piston in the cylinder, and tap it in using the wooden handle of a hammer (A). Maintain downward force on the ring compressor (B) to prevent the rings from expanding before entering the cylinder bore.

Fig 2: Positioning Piston In Cylinder And Taping In Using Wooden Handle Of Hammer
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Position all pistons at top dead center.