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I used Miracle Grow Organic Choice. You want to make sure there are no fertilizer capsules or delayed release plant foods.
It's as simple as put an inch or two of the dirt on the bottom of the tank then cap it with a couple inches of your favorite substrate. Large grain gravel doesn't work as well as smaller gravel or sand (it's harder to plant in large grain gravel anyway).
I did a video a few years back when I dirted my 5g tank. It's cheesy but if you want some video it shows the process. I didn't use the correct dirt in the video but the process is the correct idea. You may also want to remove as much of the mulch as possible. Running the dirt through a skullamackaroon (pasta strainer) helps.
https://www.youtube.com/watch?v=ly0ms5q5C9I