You have to break the block down into a grid pattern, and you have to learn how to recognize it.
The JL block is on a 3x3 grid. Like a 9Patch, but each of the patches is actually a pieced unit.
For a JL, you need five 4-patch blocks and four HST blocks.
Based on a finished 18" block, each grid unit will be 6" finished.
FOUR PATCH Block
So you need to make the 4patch blocks each finish at 6".
How big would each square be in that block?
3" finished.
So the squares (or strips) would be cut 3.5" wide - one light, one dark.
Sew the two strips together, then crosscut into 3.5 subunits, then blah, blah, blah.
HST Block
They also have to finish at 6".
The "exact" method is to add 7/8" to the finished size.
I prefer to add 1.25" and trim down.
So you would need 2 light and 2 dark squares - each 7.25" square.
Draw a diagonal line on the back of the lighter square, right sides together, sew 1/4" on each side of the line, cut, blah, blah, blah.
You either work your way down from the finished size of the whole block, or up from the smallest unit's size.