Knowing your instrument and the intricacies of organ registration is something every organist should strive for. So very good - Ann has a great goal.
The registration is a broad topic and it's best to subdivide it to various organ music genres, various historical periods, and various national schools of organ composition.
Learn them one by one. It's sometimes science and sometimes art. It's best if you could try many different organs. The person who has the most experience with the widest amount of instruments, music, styles, and national schools, wins. Listening and comparing various recordings and videos of the same piece helps a lot, too.
The first step is of course to try something new every day. That's why I send these varied sight-reading pieces every day for you to practice. Often there you can find also specific registration directions for a specific situation.
Elevazione per Organo by Floriano Arresti (1667-1717), an Italian organist and composer of the Baroque period. The registration for the Italian piece for Elevation section of the mass is usually a Principale and Voce Umana - a soft and singing 8' principal together with the undulating stop Voce Umana or Unda Maris (in the descant range) which are tuned slightly sharper and give the effect of the natural tremulant.
All Praise To Thee, My God, This Night