To get a bigger neck, you can try doing forward and backward neck extensions. Do 2 sets on each side, 5 days a week. While training the neck, you need to focus on high volume instead of strength, with explosive reps . You can also start with some simple stretches to loosen up your neck, go slow to avoid injury, and build strength gradually using increasing resistance and weight to develop your neck muscles .
If you want to speed up the process to grow bigger neck muscles faster, there are a few things you can incorporate into your regimen. It comes down to these factors: Optimizing Your Training Regimen Getting Ample Protein In Your Diet Taking Adequate Rest Between Sessions Incorporating The Right Training Tools Proper Training Regimen .
I hope this helps!