Now that your dog can wear the muzzle, and have it done up, gradually leave the muzzle on for increasing periods of time. Start with just a few seconds, and then increase in very small steps up to 1 minute and so on.
Keep your dog distracted while they are wearing the muzzle to begin with by either pushing more treats or squeezing the cheese/meat-paste through the muzzle to reward them. You could also encourage them to follow you around the house for just a few seconds to begin with as getting them moving can be a good distraction. Then remove the muzzle and let them relax. This can be a difficult stage for dogs as they’ll need to get used to not being able to back out of the muzzle should they wish.
If they start scrabbling at the muzzle or rubbing it in an attempt to remove it don’t worry, just go back a stage and carry on there for a little while longer to build their confidence before trying to progress again. Going at your dog’s pace is important, as trying to progress quicker than they’re ready for might be counter-productive longer-term.