Do you really need any hints on how to finish this Riven Challenge? If you haven't done any fishing before then read a fishing guide first.

This challenge only takes a few minutes and can be done with any Warframe you prefer. Remember to equip your Riven and also a fishing spear. I recommend you also equip some Luminous Dye in order to make it easier to aim your spear at the fish. You don't need any bait however, you can just catch some of the common fish for this challenge.

Head to Plains of Eidolon (or Orb Vallis if you prefer) and take your Archwing or run to your favorite fishing spot. Now take your time - only throw the spear when you are 100% certain you are gonna catch the fish, don't take chances with fast moving fish, only try with slow moving fish close to your fishing spot. If you miss a throw the challenge will reset - however, if you just take it easy you can easily complete this challenge in your first try.

Some variations of this challenge require you to catch X fish in a row without being downed or killed, so make sure to kill any nearby enemies first before you start fishing.