There are nearly none, other than just doing it. Apache2 gives you the right to do this, and places minimal obligations on you should you do so.
You will need to maintain the pre-existing copyright notices (s4c) and any
NOTICE file (s4d), and provide a copy of the Apache2 licence (s4a). Those source files you modify you must clearly indicate that you have done so (s4b) but to my mind adding your own copyright notice to the pre-existing one in each such file suffices, and is a good idea, anyway.
You will need to decide what licence you are going to distribute your new program under. Staying with Apache2 is probably a good idea, but it isn't mandatory. Be clear about what you are doing in this regard, and don't put the decision off until later.
There are other things, like finding a repository to host your code, choosing a new name (you haven't indicated that there are trademark interests in the old name, but I think a name change is helpful for the avoidance of community confusion), contributor interaction (mailing lists / web fora / irc channels), but these apply to all free software projects and are not peculiar to your question.