This is a video of Dice at 12 weeks of age - That's a recall! Here is how I got it:

1. I started at 9 weeks of age.  At that age, most puppies are well aware that they need you and come back quickly and easily.  Start at 8 weeks if you can!
2.  I only whistled when he was facing away and at about his furthest "comfort" distance.  In his case he is comfortable at 30 to 50 yards away (he's a confident little bugger).
3. I took his most favorite thing in the whole world as his reward - in his case, a fluffy, whippy toy.  A toy on a lunge line might be even better.  If food is your dog's thing - then start with a hungry puppy, don't let him know what you have in advance, and bring the most amazing option you can come up with - and bring a lot of it.
4. I worked in the biggest possible space I could find.
5. I whistled at the exact moment that I thought he would probably be thinking about coming back to me anyway and.....the most important part....
6. I ran my absolute fastest in the opposite direction, up the hill, through the tall grass and did my best to get as far away as fast as I possibly could.  As a general rule, it would take him at least 30 seconds to get back to me. I never even looked back to see what happened when I whistled - I just ran.
7. When he got to me, we had the party of a lifetime for at least a minute. A truly world shattering level party.
8. Repeat no more than three or four times in a session - always waiting until he's far away and facing opposite you.

That's pretty much it.  Over time I waited until he was actually moving away from me but slowing down (at a trot).  Within two sessions I started to see an instant snap at the sound of the whistle. At that point, I waited until he was cantering away. And finally, I mostly whistled when he was at a dead run, as you see in the video below - and followed the above steps. He is currently at 100% success with his whistle recall. He does not think when he hears it, he simply snaps back and around and comes back instantly. Over the next couple of months, I will slowly add a few changes. I will not run until he is part way back.  Then half way back.  Then three quarters.  Eventually he will run all the way back even when I'm at a standstill. I will begin to lessen the value of that toy, but I will continue to reward generously.  I will add some whistles when he is relatively close to me, and in some cases, I will have him do a bit of obedience work when he returns, and follow that up with the amazing party. But no matter what, those above steps?  I will make sure to do those on a regular basis for the rest of his life, just to keep the behavior as sharp as possible. Of course, your milage may vary.  You may be starting with an older dog.  You may not have a large open space to practice. Your dog may already have a habit of ignoring your requests to return.  You may lack a potent motivator - so many possibilities!  I cannot coach you through all of those possibilities, but I can say that the more of the above steps you can put into practice, the better your odds of having a super strong recall even under distraction. Have fun! https://youtu.be/0KR-L66zA58