Language Change

Currently, I've embarked on the mammoth task of creating a number of
descendant languages. I already have the phonology of the parent
language worked out but I have no idea where to go from there.

Actually at this point, I'm only concerned with creating the lexicons
for these languages and some affixes relevant to place names. Grammars
(if possible) I'm hoping to form later on.

If anyone can help me with some advice on where to start with phonology,
I'd appreciate it.

Do I create all the phonological rules for these descendant langs
completely from scratch and apply phonological change, or do I form them
from the parent lang?

I have been using the SPE system to map the parent lang and I've been
having difficulty trying to determine how subtle or obvious the changes
should be.

Also, what other considerations should I have with language change with
respect to morphemes and words in general?

Thanks,

Michael