McP77

I personally wouldn't go tile on tile in a shower area. You don't know what the substrate behind the existing tiles is like. If it hasn't been tanked then it could be in bad condition and then you are adding the weight of extra tiles/adhesive/grout onto it.

That said it doesn't answer your question. Lay the job out as you would any other - make the best use of full tiles and keep cut tiles as large in size as possible. Try and avoid lining up the new grout joints with the existing ones.

You don't mention tanking in your prep. After cleaning existing tiles, when dry skim with flexible rapid set and apply WP1 tanking onto that (you can't apply tanking directly onto existing tiles).

After grouting leave as much time as you can before applying the silicone.