This is a great series to watch, these are done by Nathan Lively:
Avoid doubling up electrical and acoustical filters
How to choose starting filters for crossover alignment
How to choose crossover filters for matching slopes
Set driver gain and filter cutoff to match crossover target
This last video takes some type of software that can measure phase, SMAART is great, but Open Sound Meter is similar and free. You could do this just by looking at the combined RTA results to get a 6db summation to a certain extent.
How to use delay to match slopes in your crossover alignment
Some Others:
How to use Auto EQ in REW
Should you use band limited pink noise as your test signal (for overall measurements)