Passive crossovers are the hardest part for a DIY speaker. Getting them right can be challenging. It isn't as simple as using a calculator because calculators cannot predict the acoustic output performance of the speaker with the circuit it is designing. LEAP and other professional speaker design programs can predict the performance fairly well, but even then a little experimentation is needed to get the sound perfect.
I guided a few forum members through designing their own passive crossovers with success. It can be done and you will learn as you go along. But, I recommend using active crossovers as the performance will always be significantly better and you can alter the settings infinitely and whenever you want.