How do you treat blisters in the mouth?

Depends on the cause. The first step is pain control. Most find benefit from avoiding spicy and acidic food and drink. You might also avoid things that are crunchy in favor of soft foods. Some people get benefit from over the counter pain relievers. There are many things that can cause oral blister or ulcers, usually viral infections. However, you should visit your doctor if they arose after you took a new medicine.
Depends. It depends on what's causing them. Many are just caused by a variety of viruses that we have no specific treatment for. if this is the case then you just treat the symptoms - usually pain. If it's herpes related you can use antiviral therapy like acyclovir. There are other causes too, like nutritional deficiencies. You should feel comfortable addressing your concerns with your doctor. .