Your dog stops at the same patch and starts nibbling. It is easy to assume they're trying to make themselves sick, but many dogs eat grass without being ill beforehand or vomiting afterward. Some enjoy it. A sudden, urgent change is a different story.
Notice how much they eat and how they behave after. If grass eating is new, frantic, paired with repeated vomiting, diarrhea, poor appetite, or discomfort, contact your veterinarian. Ask before changing food or adding fiber or supplements. A dog eating grass occasionally while otherwise well may not need a training fix.
Choose where they graze
Keep your dog away from lawns that may have pesticides, herbicides, or other chemicals, and from areas contaminated by animal waste. On walks, use a secure lead and choose cleaner routes. If your dog lunges to gulp plants, call them away early and reward moving with you, don't yank plants from their mouth if they guard objects. Learn which plants are safe in your own garden and keep toxic ones inaccessible.
If you want less grazing, offer time to sniff on a safe part of the walk and practice an easy "this way" cue away from grass. Reward your dog for following you past the patch. Avoid turning every blade into a struggle. The important distinction is between a dog calmly nibbling known-safe grass and one who is suddenly eating urgently or becoming unwell.
Sources: VCA Animal Hospitals: Why Dogs Eat Grass. Dogs Trust: Keeping your garden dog-friendly.