Please look at your product's dosing instructions first, but one recommended dosage of fenbendazole for use in food is 2.5mg per gram of feed. Feed the medicated food for 2-3 days, then repeat that feeding regimen again in 14 days. So what you would do is get or prepare the amount of food that these fish typically eat in 2-3 days, weigh that, then mix in the appropriate amount of medication. A gel food would be easiest. Ideally, you want to prepare only the amount of food that is normally eaten in the given time frame, to avoid waste and to ensure appropriate dosage.
You will also want to take into account any additives in the medication. The dosage listed above is assuming 100% fenbendazole is being used. If your product contains other additives and you still follow dosing that assumes 100% medication concentration, then you will end up dosing less medication than you intended and may not see results. Because of this, I also recommend reading any instructions and dosing guidelines that come with the product you've purchased. You can compare them to the dosage rate above, if the product provides instructions for dosing in the feed.
You may have to do some additional calculations to ensure proper dosage if your product is not pure fenbendazole. If the product has other ingredients and does not list the concentration of each ingredient (on the package or online), then you should default to the package instructions because in that case, you won't be able to accurately calculate an appropriate dosage yourself.
Edit: I just saw that your fenbendazole granules are listed as being 22.2% fenbendazole, so take that percentage into account when you calculate dosage. For example, if you were to prepare 50g of medicated food according to the dosage listed above, you would need 125mg of 100% fenbendazole. But because your meds are only 22.2%, you would need 563mg of your granules to get 125mg of fenbendazole. I recommend getting an accurate scale if you want to use this method of dosing! If you can get a jeweler's scale, those are best because they're meant to measure small amounts of weight with accuracy.
As always, please compare this with your product's instructions! The dosing I'm using here is just one recommended dosage out of a handful, and it is out-of-context for this particular product and these particular fish (most fenbendazole dosing recommendations are actually for food fish like trout and catfish). I'm not a vet, so I can't guarantee that this dosage is the most appropriate for your specific fish. We typically have to be the ones to play vet for our own fish, and that comes with a certain amount of unavoidable risk.