Hello!

As in the title, I have found both of these items but can't seem to figure out how to use them. I've clicked 'use item' for both, and now both are in my 'garden supplies' section, but I have no idea what to do with them now.

you go to your garden area, select the plant you want to use them on, and use them (if you have enough of them)
You can only use as many as the fertiliser used on the plant allows.