Basically Babylon.js comes with a plugin already defined for cannon.js
You can create your own plugin by creating a class that provides the following interface:
function initialize(): Must initialize your engine
function setGravity(gravity): Used by Babylon.js to set the current gravity
function runOneStep(delta): babylon.js will call this function for each frame, giving you the delta time between current and previous frame. This is the responsibility of the plugin to update meshes' position and rotation accordingly to the physics simulation.
function registerMesh(mesh, impostor, options): babylon.js will call this function when the user wants to create a physics impostor for a mesh. options parameter contains 3 values: mass, friction and restitution. Possible values for impostor are the following:
function registerMeshesAsCompound(parts, options): Babylon.js will call this function for compound objects. parts parameter contains an array of {mesh, impostor}. options parameter is the same as above.
function unregisterMesh(mesh): Called to remove a mesh from the simulation
function applyImpulse(mesh, force, contactPoint): Apply a specific force to a specific contact point for a given mesh
function createLink(mesh1, mesh2, pivot1, pivot2): Create a joint between two meshes
function dispose(): Free all allocated resources
function isSupported(): This function will be called by Babylon.js before everything else to ensure that your plugin can be instanciated (You have to check if required .js files are included for instance)
For implementation details, you can refer to cannon.js plugin: https://github.com/BabylonJS/Babylon.js/blob/master/Babylon/Physics/Plugins/babylon.cannonJSPlugin.js
When you launch the physics simulation, you can add a new parameter to enablePhysics function to indicate which plugin to use:
scene.enablePhysics(null, new BABYLON.CannonJSPlugin()),
The first parameter can be used to define gravity (which is (0, -9.82, 0) by default).