Short version
Requirements:
Summer season
Morning - Noon (6:00Am - 12:00PM)

Fishing Rod (>= Copper)

Also short version but in italic

To join in Secret Hideout (Cooper Mission) you need to find Marlin(A fish) for Cooper so all you need to do is
1.waiting for summer season
2.fishing at the sea around 6 AM-12 PM

• Buy bait for easily fishing
  
• Use upgraded fishing rod (Copper or iron will be fine)
  
• Marlin will has a large shadow when he trying to eat your bait

3. Tadah!! you got Marlin

4. give it to Cooper to interact cutscene

• DO NOT GIVE IT TO SANDY or you will lost the fish

(ภาษาไทย) Thai languague

วิธีจับปลามาลิน
1.ก่อนอื่นเลยต้องรอให้ถึงช่วงฤดูร้อนก่อน
2.เอาเบ็ดตกปลาไปตกที่ริมทะเลตรงขอบร้านค้าของ Sandy
3.ซึ่งวิธีสังเกตปลามาลินนั้นจะมีเงาปลาใหญ่กว่าปลาตัวอื่น ก็จะได้ปลามา
4.ห้ามส่งให้ Sandy ต้องเอาให้ Cooper ไม่งั้น Cutscene จะไม่ปรากฏ

Watch the video for more information

Dig secret Hideout with hoe

• Go outside the cave
  
• use hoe with secret hideout

Hit the Big Tree's roots with axe

• Go to Big tree root
  
• Hit it with axe
  

Dig out big skeleton's hand

• Go to Swoosh Falls
  
• Visit Large fossil
  
• Use pickaxe at his right hand

Hit ruin's pillar with pickaxe

• Go to Cistron Ruin
  
• Hit the ruin in the picture

Scrub second stone with brush

Hit many rocks with Hammer

• DO NOT HIT BOULDER IN YOUR FARM. IT'SNT ROCKS
  
• Go to Plash River
  
• Find the magical rock
  
• Hit it with Hammer

Edit: I forgot to add scrub the twon name

it's in the square.