Relax and unwind. The first step in learning to visualize (or paint a picture in your head) is to de-stress. You'll need a lot of concentration so ensuring that your mind is worry free and relaxed is important. Try relaxation techniques such as having a bath, listening to chill-out music or enjoying a massage.
Use a visualization or meditation CD. When you are learning to paint a picture in your mind for the first time, it helps to have someone talking you through the process. One option is to buy a guided visualization or meditation CD which will feature a soothing yet instructive voice talking you through the visualization process. They will use phrases such as "Imagine you are..." and "Now think of...". These sentences will clear guide your imagination and help you start to achieve a mental image.
Attend a meditation session. As already mentioned, learning how to paint a picture in your mind requires full concentration and your body and mind to be in a relaxed state. Meditation classes teach you how to unwind and, like a CD, will guide you through the process of relaxation and imagination - live. You can find local meditation classes by searching Google.com using the search term 'meditation class' followed by your town or city.
Revisit memories. Find a quite spot to simply sit and think. Try and revisit old memories in as much detail as you can in your mind. Think of a time and place and paint the picture in your mind of exactly how it looked and felt. Consider: what you were wearing, who was around you, how they looked then, what the weather was like, what colour the sky was, etc. If you can't remember the event accurately don't be afraid to imagine new details as part of this exercise.
Read more. Spending time with a book (preferably fiction or poetry) is an excellent way to improve your ability to paint a picture in your mind. Focus on the visual descriptions provided and work hard to visualize these details internally.
Day dream whenever possible. Learning to paint a picture in your mind takes practice. Try and day dream whenever possible, practise visualizing an ideal afternoon out or an event that you are really looking forward to. Painting a picture in your mind allows you to conjure in details any fantasies you may have. Don't forget to add as much detail as possible to your mental imagery. Visualization isn't hard, it just requires you to accept the power of your imagination and have fun with it.