RETILE BATH FLOOR & SHOWER/TUB WITH PORCELAIN

I have 20 sq/ft of porcelain tile on the floor & ceramic tile in the tub/shower. I want to put new porcelain tile in all areas. Questions: 1. ) how can I tile over the porcelain floor tile without removing it,I.e., what is the procedure and what materials do I need. This tile is in excellent condition 2. ) after removing the ceramic shower tile what procedure & materials do I need to treat the old sheet rock before installing the new porcelain tile.
Thank you very much,
Richard

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