To be able to access the Fishing Function, you need to be on level 26 and you have to explore/unlock the Fishing Area (Located near the Captain Walker D. Plank)
You can catch fishes in the Pier (red circle) and access the Fish Order by clicking in the order house (blue circle).

Fishing Function

1.  Choose or craft a Bait by clicking on the Bait icon.
Tip: You can choose multiple Crops and Fruits at a time just by clicking them one by one.
 
2.  Press the button “Start Fishing” and you will be able to see the time left (If you wish, you can click on the timer to skip the waiting time with Rubies). You can Fish more than once at the same time, you can add more fishing slots by clicking on the slots with a “+” icon and building it.
Tip: If you can’t fish, check if you have enough Baits.

3.  Once the fishing is completed, simply click on the Fishing Floats icon to get your rewards. You will get a fish (You can try different Baits to get different fishes) and FXP (Fishing Experience – Blue Star). You can check your FXP by mouse over the FXP bar, once it’s completed, you can access the next Fish page with new fishes and better rewards!  Just craft the Hook needed for this new page and you will be ready to your new challenge!

Fish Order

You can check in the Fish Order the orders available to sell.
If you wish, you can click on the button “Turn Down” to cancel an order, if so, a new order will appear in few minutes.
Make sure you have all needed fishes in stock and you can click on the Sell button to sell.
After sell your order, the boat will sail away and back in few seconds with your rewards, simply click on it to collect.
By completing Fish Orders you can get Coins, XP and Pences that can be used to purchase special items. You can check these items by clicking on “Spend Pence”.