Take a look at this screenshot below. It will show which to click ...
Firstly you need to double click on the players you want to make home grown status so they come up in their own tab. And then for each player click on ...
Then click the SAVE Button after you have done this for each player.
Then click on the RELATIONS tab. And change any instances that show anything about another club. Click SAVE again for each player.
You will have to progress 1 Day in the saved game for changes to take affect/effect.