Your only real job is to watch, from a distance
Most found-snake emergencies go wrong at the moment a well-meaning person decides to do something with their hands. Put that instinct down. Stay several paces back, move children and pets indoors, and keep your eyes on the snake so you can tell the handler where it is and which way it went. A snake left alone almost always stays put or slides quietly toward cover; a snake being followed, filmed at close range or prodded with a broom is a snake making fast decisions. Unless you can identify the species with complete certainty, treat it as an animal you do not touch: even harmless species bite when grabbed, and escaped pet snakes can look deceptively similar to species that are not harmless at all.
If the snake is inside, contain the room, not the snake
When the snake is in your house, garage or shed, resist the urge to catch it and instead make the room its temporary enclosure. Usher everyone out, close the door, and push a rolled towel firmly along the gap underneath so the snake cannot slip into the rest of the building. Then leave it completely alone: no checking every five minutes, no flashlight beams, no attempts to tempt it out with food. A calm, undisturbed snake in a closed room is a straightforward pickup for an experienced handler, while a stressed snake wedged behind pipework because people kept opening the door is an afternoon's work. Photograph it only if you can do so from the doorway without entering its space.
Call the right help and describe what you saw
The people who do this well are local reptile rescues, wildlife rehabilitation groups, animal control services and experienced keepers, roughly in that order depending on your area. When you call, describe the snake's rough length, thickness, colour and any pattern, where it is, and whether it is contained; a clear photo taken from a safe distance helps them identify it before they arrive and bring the right equipment. If the snake is outdoors and clearly an escaped pet in a garden, keep watching it from indoors if you can, because the handler's job is far easier when someone can say it is still under the third bush from the gate.
Resist the well-meant shortcuts
Do not try to pin the snake, scoop it into a bucket, or usher it down the road with a rake: those are the moves that produce bites, escapes and injured animals. Do not release it somewhere wild either, because an escaped pet snake is unlikely to survive and can harm local wildlife. Knowing who to call before you ever need them is the real preparation here. Browsing KinsLe's Pet Services Directory for reptile rescues and services near you means that when a snake turns up, the number of someone experienced is already in your hand.