I've had this wooden tray for years but the varnish had worn through. It was beginning to be the worse for wear as I use it every day. Recently I found some cute wrapping paper which I thought would be lovely for decoration and so I thought I would try decoupage. As it was my first attempt I wanted to give it my best shot and do everything by the book. You could I suppose miss out the primer, if the tray isn't in too bad condition, and I guess one or two coats fewer of varnish wouldn't hurt. That way you could possibly finish it in one afternoon, with the aid of a hairdryer.
If you want to have a go at decorating a flea market find, this is how you do it.

You will need:

• MDF or wood tray
• pictures cut out of paper
• water-based primer (optional but may produce better results)
• paint (I used Farrow & Ball vert de terre)
• water based satin varnish 
• raw umber powder pigment or raw umber artist's acrylic paint
• paper towel 
• sandpaper (wet and dry causes fewer bits)
• soft cloth

What you do:

1. Wash off any grease from the tray.
2. Sand all the tray to smooth it and to provide a key for the paint. 
3. Apply two coats of white water based primer paint, very lightly sanding after each one.
4. Apply two coats of  paint in the colour of your choice (I used some left over from painting my dining room).
5. Cut out, arrange and glue you pictures onto the tray, taking care to make sure there are no air bubbles.They can overlap.  At this stage, after they have dried, you can spray the pictures with fixative to prevent the cut-outs from bubbling - I didn't.
6. If you wish to give it an antique look, mix three parts raw umber pigment with one part white primer paint mixed with eight parts water. Brush this onto the tray and quickly wipe it off with paper towel, before it dries, leaving the paint in the crevices and brush marks.
7. Brush on at least four coats, I used seven, of water-based satin varnish, brushing in different directions each coat, sanding lightly between each to remove bumps.
8. After wiping to remove dust, finish with two coats of polyurethane matt varnish to seal the tray.