I'm also looking for info on this topic! Any input would be appreciated.
I did my undergrad degree in biology and the path to grad school seemed more straightforward: intern in a lab, do a research project for your thesis, get a job working in a lab, gradually focus coursework in on a field, work in a lab aligned with your interests, find phd programs that are strong in that area.
In education, it seems people come to PhD programs from a wider variety of places--teaching, industry, administration, etc. and it's not as clear to me what makes a strong app. (It's also really hard to google because the keywords "education" and "PhD" don't really exclude PhD's in all other subjects.) Faculty members seem to research a wider variety of topics as well--unlike biology professors they don't specialize as exclusively. Advice on what leads to successful applications?