Set a project folder and save your scene.
* If your painting into an existing texture you can skip this step.
Make sure your model has UV coordinates.
Make sure your model has a basic Lambert material.
* If you’re using a different material/shader on your model,
Temporarily switch to Lambert just for the texture painting operation,
And connect the original shader with the painted texture map to the surface shader input of the shading group after you’re done painting.
In the Rendering tab toolbar, Double-Click the 3D Paint tool button to activate 3D Paint and also open its tool settings window.
Make sure the object is selected.
In the 3D Paint Settings window, go to the File Textures part,
Choose a material attribute to paint to.
* this would usually be Color because we are using Lambert temporarily anyway.
Click Assign/Edit Textures to open the Assign/Edit Textures dialog,
Choose a resolution and a file format for the new texture,
And than click Assign/Edit Textures button at the bottom of the Assign/Edit Textures dialog to create the new texture and close the dialog.
* If you already connected an existing texture file to the Lambert shader’s Color input you can skip this step.
Check Update on stroke and Save texture on stroke.
Set the viewport to Textured display mode.
Set paint brush settings.
Click the 3D Paint tool button to activate it and paint on the model surface.