In medical terms, cold sores are called herpes simplex 1 and usually appear on the lips. Some are unlucky and gets breakouts inside the mouth or up the nose, but the vast majority get breakouts on and around the lips. The outbreak starts with a “shiver” or a kind of tenderness in the skin. Then small clear blisters appear in larger or smaller clusters. This is the time to be careful because the liquid inside the blisters is full of contagious herpes virus. You can carefully poke holes in the blisters with a sewing needle that is cleaned with alcohol. Then you gently can squeeze the liquid out of the blisters with a cotton ball/kitchen roll. Pay close attention to washing both hands and needle thoroughly afterwards.
Don´t kiss anyone if you have an outbreak of herpes because it is very easily transmitted. The vast majority of the infected have been infected before they are 5 years old and they have usually been infected by a family member or in their kindergarten/nursery. Herpes/cold sores are most contagious in an outbreak, but you actually can infect others before you even know you have an outbreak on the way. Likewise you can get infected by others before they even know they have an outbreak coming up. Because the virus is that contagious it is almost impossible to stop the chain of infection.
The only thing you can/must do, unless you’re completely paranoid or move to a desert island or never kiss anyone, is to be very aware of when you get outbreaks of cold sores/herpes. React at the slightest sign of herpes and avoid kissing. Start treatment as soon as you feel an outbreak coming on and continue until the sore is crusted. Wash your hands often and do not touch the sore. Avoid direct sunlight on the sore and frequently apply the gel you use against herpes. Wait until the crust has gone and then you can kiss again all you want.