I fertilized my lawn for the first time. I borrowed my neighbour's drop
spreader. When I returned it to him he asked if I did a perpendicular pass
when applying. He mentioned that I would get lines in my grass...where the
fertilizer would have been missed.

Well. I didn't realize that spreading fertilizer is like painting a wall.
You would have to stare for a very long time to find the art in the design
that I left (my yard shape is irregular). I thought that the fertilizer
would spread in the soil after a rain...actually, I wasn't thinking...

How much time should laps before applying more fertilizer?

Matt