My students use my detailed 4-page "Come" training plan to train their dogs to come to them using the cue "come". It's very helpful to ALSO train this same behavior using a whistle. Why? Because if you're like me, your dog goes with you to the beach, or to run in fields, or to a large dog park, where you would have to really yell your verbal cue in order for your dog to hear it. So having 2 cues for the same "come" behavior, one of which is handy to use in noisy, windy, surf environments, is very helpful.
Remember that it's OK to have two separate cues that mean the SAME thing. That is not confusing to your dog if you train the cues to the behavior the same way. In our own language, we understand "stop" and "halt" to mean the same thing. What is confusing is when we accidentally let one cue mean two DIFFERENT things to the poor dog. If "down" sometimes means "belly on the ground, please" and other times means "put your paws on the ground", that's confusing. That's why in my house, "down" means "belly on the ground" and "off" means "put all 4 of your paws on the ground". Each cue should only mean one thing, but it's OK to have multiple cues mean the SAME thing. Make sense?
OK. So get yourself a whistle. If you are one of those fortunate people who know how to whistle really loud, and you can still do that in wind, you don't need a separate whistle. I have a few whistles because I cannot whistle loudly on my own.
Get some HIGH VALUE treats, and play the backup recall game with your dog, where you hold a treat at their nose level, and backup quickly as you toot the whistle. The dog follows you, and you release the treat. If you can handle it, CLICK with a clicker and then release the treat. So the whistle stays in your mouth the whole time. You back up again, tooting the whistle, dog follows, you click and give the treat. Do this about 25 times. Quit.
Next time, pick a moment when your dog is about 10 feet away sniffing something, and (having the high value treat ready), toot the whistle, and click/treat when the dog comes to you. Slowly build distance, and in other situations reduce distance and slowly build the dog's ability to do this is slightly distracting environments. Raise the "bar" of distance and distraction separately. And when out in public working on this, use a long line.
Never ever stop practicing your dog's recall. Keep it happy, reward it generously, and keep it fluent.
I bought about 3 whistles on sale recently for $2 each. The model I bought normally costs $9 at a major pet supply store (they were on sale). I jumped at the chance to buy them when I saw their price. I know I'll need them over time, and I'll keep one in my car, one in my dog-treat fanny pack, and one at home. You don't have to spend a lot on these. I have also used a little combo compass and whistle (for hiking and emergencies) that I got at REI one year. It's got a great little whistle.