1. You will have to fish at least once in order to get the highest quality of Bait so that it is much more easier to catch the Guardian.
2. Once you fish, there will be a chance that a yellow/Golden fish will spawn after a couple of tries of fishing. The silhouette of the is slender yet slightly long. It may be easier to identify if you have read the fishing book that will allow you to use your Third Eye during the fishing session.
3. Once the Guardian has been identified, carefully throw your bait at the front of the fish but a bit further away from the mouth. Once it goes for the bait, you have to recast quickly. You objective here is to have the Guardian get as close to the place where you are sitting at. The reason for this is because if you try to catch it while it is far, it may be near impossible to reel it in because of the limited space you can press X(reel).
4. The trophy will shortly appear after once you obtain the Guardian.
-- Things to be wary of --
*Be careful of the nearby fish as they will also be attracted to the bait