Step 5: On the spot assembling and trimming
Assemble the parts as shown in the picture. Put the sub in the water. Make sure the motor is filled wit water before starting to trim. Either tap on it with any holes in the motor positioned at the top, or if possible place it under a running tap with one of the openings. A little air left in the motor is not that bad, but to much will make you "overtrim" and your sub risks not surfacing anymore once the air finally does escape from the motor when running.
The last picture shows how to trim (I did it in a bucket for convenience). Once the solar panel catches some sun your sub will run, for the moment remaining at the surface. To dive check out the next step. If you accidently "overtrim", having the sub dive/sink even without running forward, you can always add some floating material back again. Glue it with hot melt or a little super glue, or slide it firmly under the "clip" formed by the solar module.