How to texture paint your mesh in Cycles Render
1. If you have a simple mesh you may want to skip this step, otherwise I suggest you to go to EDIT mode and select the part or connected parts which you want to paint separately. Press P, a menu will appear asking you which option to choose (Selection, By Material, By loose parts); select Selection. Now to select either the original mesh or the newly created object you have to go to OBJECT mode and then return to EDIT. From now on, each part of your mesh will be dealt with separately.
2. If your object is complicated, I suggest you to MARK THE SEAM otherwise leave it.
3. Split the screen in two parts and on the bottom left of one of the panels, click on the little cube, select UV/Image Editor.
4. In the other panel (3d view), select the object you want to paint first then switch to EDIT MODE. Remember that for each panel to be active, your mouse must be hovering it. Press U to unwrap. For a very simple object, Smart Unwrap (no marked seam necessary) is very convenient otherwise just choose Unwrap.
5. Return to the UV/Image Editor panel and at the bottom you will see the icon of an image with NEW next to it. Select NEW and choose a name for the object you are about to paint. Make sure to type a name you can easily associate with the object. Once done, you will see that your object has turned black just like the background color of your UV map. This is because the two are connected so don’t panic! You may also need to zoom out to adjust your view.
6. Now, on the right side of the 3d view, select Material then New and give it a name by double clicking on Material.001. At the far end of the Color section, there is a dark gray area. Click on it and select Image Texture, right under it a new menu will appear. If your click on the image icon, your newly created UV map should be there, otherwise choose open and select it manually.
7. Remember to save your work. We are now ready to paint, hover your mouse over the 3d panel and select TEXTURE PAINT. In the 3d view panel, your object is still black at this stage, press T to bring up the Paint tool bar. Select the color you want and start painting your object. You can adjust the radius and the strength of your brush from under the color palette viewed as a colored circle.
8. As soon as you start painting, you will notice that your UV map gets updated in real time. It is fundamental now to save your UV map as Image. This is the image that you will keep forever even for future references so place it somewhere easy to find and to connect with the original image. Later on, if you want to export your object, you will have to export its related images too. The colored UV map will be saved as a PNG file.
9. Select Save as Image from the UV Editor and save your image as a png file. Every time you make changes to this object, an asterisk will appear on the menu next to Image to inform you that you need to save your work.
10. Back to painting your object, you will notice that the brush may not be as accurate for some little corners. In this case I suggest you to paint directly on the UV map. On the bottom menu of the UV editor, click View and select Paint. Now you can paint directly on the UV map and at the same time, you can see the result in the 3d view. It’s often good to switch between the two.
11. Once you are done, repeat the process for the other items connected to the object which we separated earlier.
12. When you are finished painting and you are satisfied with your work, you can join the parts together again so that you can move them around. Select the small objects first and the main mesh last by right clicking on it while holding SHIFT down. When all the objects are active, press CTRL + J to join them. However, if something goes wrong you can again split the parts with the Separate option as explained earlier.
13. Texture painted objects are often very dark when rendered. In this case, once you have arranged the lighting appropriately, there is another tweak that you can use to balance the light.
14. Go to the right hand bar in 3d view and select the world; it’s the icon with Planet Earth. Select Ambient Occlusion and adjust the factor. You can see the changes in real time if you select the Viewport Shading to Rendered.