....Like you, I am a bit carpentry challenged. I have no idea if I built this stuff right or not but it's been holding my stuff for a few years now with no signs of any change / sagging / etc. It has held several hundred pounds of boatanchor gear with zero issues.
If it's been holding the stuff, you've built it good enough. The main point about any sort of home made bench is that it does what is intended. That's what is important. Now, if your intent was to build it to professional standards, then you could worry about whether it was 'built right' or not!