This is one of the more challenging tasks to teach a dog. At 6/7 months old I would not expect your dog to stop to the whistle unless you have been teaching it to from an early age. This is not easy to explain, much easier in person, but here goes.
Put your dog on the lead and walk to heel, everytime you stop blow the stop whistle and tell the dog to sit.
Do this for about a week, 3 or 4 short sessions of 5 minutes per day. Once you think the dog is getting the hang of it, dont use the voice command.
Complete the above without the lead. Walk your dog to heel, blow the stop whistle and continue walking. 9 times out of 10 they will continue to follow you. So this is where you have to be quick, as soon as you have blown the whistle check to see if your dog has sat down, if not grab them quickly and put them back in the place you blew the whistle. Then after a couple of seconds blow the stop whistle again to confirm your action. Walk away from your dog five or six paces, and then go back and praise for sitting still. After about a week they will sit while you continue walking.
The stage after this is really to let them have some free time running round, but when they are quite close blow the stop whistle, if they stop and sit, praise with your voice, then through a retrieve to the opposite side of you that the dog is on, if the dog still sits send for the retrieve.
I hope that makes sense. there are other exercise you can do after the dog sits to the whistle at your side.
best of luck.
Ian