What kind of snake? For Morelia, here's what I do...
1. Use a cotton bud with one cotton end cut off to gently open the mouth by rolling outward under the upper lip. When the snake opens up, move the bud to the back of the mouth to keep it open.
2. Hold a warm, damp pinkie mouse behind the head with blunt forceps (can dip mouse in egg or oil if you wish, but water from thawing is my usual) and place this firmly in the snake's open mouth.
3. If possible, have someone else pull the cotton bud out of the side of the mouth so that you can move the head of the mouse down the throat of the snake. Keep your thumb and index finger gently on the upper and lower jaw so that it cannot regurgitate the pinkie.
4. Keep shoving the pinkie down the throat. It will probably 'pop', but carry on, shoving as much in as you can until you can close the jaws over the meal.
5. Now start to gently massage the meal down the snake's throat until you reach the stomach area and the snake stops trying to regurgitate the meal.
It is always best if someone with experience can show you what to do. This is a real juggling act, because you have to control the snake's body with the rest of your fingers and the snake is resisting with all of its might if it is in any sort of reasonable condition. It will evacuate its bowels and bladder on you, wrap around the hand holding the forceps, etc. This is not a procedure to be undertaken lightly.
When a hatchling is a bit older I can usually get them to open up by pressing the head of the food item against their mouth, but the cotton bud trick is a good stand-by. If you are dealing with a smaller species such as Antaresia, you may need to use a tail, but these have very little nutrition. A whole animal is much better. Before going through this process, ALWAYS offer the meal as a regular feed first. Assist feeding is a last resort.