Sharpening a knife is a multi step process and there is more than one method.
The first thing to know is you knife/tool sharpened on one side or both sides?
Then look at the edge for nicks and dings if there are any, use a second cut file and remove them. If you edge is on both sides work each side equally, or work on the angled side if the blade is sharpened only one one edge. (remember the flat side of a single edges are NOT touched at this time)

The knife edge has two angles the relief and the edge, when we sharpen we work with the edge only in most cases. (Restoration is a different topic)
The act of whittling emulates the correct angle for most knifes, measuring is not needed.
You can use sandpaper if you like to get started (I don't) I use Arkansas stones but there are other good ones out there.
First a soft one
Then a hard one
and finish with a black one (fine) There is a stone finer than this but for gardening and grafting would be over kill.
A hand loop or magnifying glass is helpful for beginners to see the burr. Keeping your stone well oiled to remove debris will help immensity, there is nothing worse than trying to sharpen with a plugged stone!
Apply an even pressure on the blade at the whittling angle draw the blade from tip to the base while crossing the stone from end to end. (remember to keep both sides even for double edged blades. Work with the first stone until the edge creates a curl this is the burr stage and indicates time to move to the next stone.

At the end the final burr can be removed with a piece of leather, on both sides this time if you had a single edge.

Randy AC7NJ