When do you use alternating plant patterns, layering and/or grouping plants? Not sure if this question makes sense...

Sorry, but I don't understand your question. Please be specific

I have seen gardens to where they have used two or more plants and planted them in rows in an alternating pattern; or plants are grouped together and planted along with other group of plants creating a layered effect--is there a guideline on where these "methods" best to apply on? I hope this clarifies the question above.

There's no easy answer. It depends on whether I'm using the plant to create a sense of repetition (rhythm) or as a single specimen for a statement/punctuation. I might be trying to relate the planting design to the architecture - or the topography. A contemporary design might call for Mondrian-style geometric or linear plantings but an informal design might need interconnecting sweeping waves. It takes years to become a good designer - and even then so much is intuitive. I'm sorry this probably isn't a helpful answer!!

You might enjoy attending some seminars on garden design, reading books or watching videos - have fun