This is a hard one to tell anyone that has not done it b/4..
You 1st. need to find the angle that's to say how the X will cross in the center and your side parts will set this angle up and you need to have it set b/4 you can cut the X laps.
Most of the time the cross bars will go into the corners of the frame, that's to say they are cut with 2 45deg. cuts so they fit in the corner, this sets up the angle of the X lap cut.
Once you have the cross bars cut to the right size you would lay the cross bars on top of ea.other and mark the one on the bottom and the switch the parts and do the same thing... now they should be marked with the right angle, because this is not a true 45deg. mark, you can't just use a tri.sq. to mark the boards for the lap cuts.
Because it's based of the center of the boards and not the sides of them.
It's best to use a radio arm saw to make the dado cuts but you can make a jig and use the router and for a nice clean job you can put a 1/2" long teton lap loint in the corners this will lock all the parts in and this is good job for the router.
Hope this is what you are taking about, see snapshot below ▼