Well, there's three things you have to know about. After that, it's pretty simple.
Each FRAME has different natures and IVs.
Each SEED is a string of frames.
You can hit any seed by getting the correct DELAY.
To put this all together, let's say I'm catching a legendary at frame 40 on seed X. To land on seed X, I have to hit a delay of Y. So if I hit a delay of Y, I'll be on seed X. After that, I just have to advance to frame 39 and talk to my legendary, which would then have the spread of frame 40.
That's basically the short version, it's easier for me to help when you're actually trying.
Do you have a legendary in mind that you wanted to try?