The very first thing you are going to have to do is to wash those chairs and tables to be sure that there is no dirt or dust on them. If there is either the paint is not going to stick and it will leave an uneven surface which is unwanted. The more careful you are with the cleaning process the better results you are going to get.
Wooden items, such as picture frames, shelves, furniture, and mirrors can be made new with a fresh coat of paint. I have done this with several items in my home and have been thrilled with the result. For picture frames and mirrors, remove the glass from the frame before starting your project. Start by sanding the wood just a little with a piece of fine sandpaper. Sand the entire surface using light, gentle strokes. Roughening the wood up just a little will help the paint stick to the surface. Don't damage the wood by going crazy with the sand paper. There's no need to completely remove the old paint or finish.
After this you are going to have to use sand paper for every part of the furniture to make sure that the shiny surface is evenly worn out from every part. When you are done you should wash the furniture again and let it dry. Only use paint that has been designed for plastic use. Carefully apply an even and a thin layer of paint and let it dry. Once it is completely dry you should apply another layer for better results.
The last step of preparation is to paint a piece of cardboard or watercolor paper with the same color as your background. Once that dries, place the stencil on the prepared paper and tape it with painters tape so that it doesn't move. Don't forget to match up the registration marks if you're using a stencil with multiple layers. Select the colors for each part of the stencil and begin. Just like using a stencil brush, you want only a small amount of paint on the sponge. Once you have dipped your sponge into the paint, wipe it on the sides of the container and dab the sponge on a paper plate or paper towel a couple of times. You don't want the sponge wet. It will appear dry but still leave color wherever you dab it.