It can be a still life (I rarely use this form but it is very popular) or your favorite room somewhere, something from your mind or other subject matter to work from.  I do my painting 99% of the time in my studio from reference material (mostly my photos).  My photo will be my value study, color and design study.  Photoshop does wonders for me.  There are other programs that are just as worthy.  My discussion here is from that viewpoint. 

Go through your own reference photos (cannot use others, it is a copyright infringement) or a place that you can design out on paper first.   I keep a file of watercolor worthy photos by subject to choose from. Try different sizes of the same subject, perhaps cropped in some way to get the effect you are trying to achieve.  I Use this photo to put the painting together.  Frequently I use a projector and trace my painting onto my 300lb paper.  This old method was used by the masters with lens and boxes!  There are other ways, this works best for me. 

How is the photograph’s design?  Is it lopsided?  Too much interest on one side?  Does it flow (does your eye move around or do you get stuck on a spot?) Does it have a pattern?  (T, S, L or other letter) Each painting should have some of this in it.  Make sure that the focal point of your painting is in one of four spots on your painting.  (put two lines, horizontally and vertically, equally spaced on the photo you are using.  Where the lines cross should be your focal point at one of the crosses.   

Values:  Your darks and lights should be next to each other.  Contrast adds drama to a painting. 

Are your warm and cool colors in the right place?  When you have a shadow, it should be a cooler color, in direct light, it should be warmer color.  Using Red:  Warmer reds should have some yellow.  Cooler reds should have some blue.  Using Yellow:  Warmer yellows should have some red.  Cooler yellows should have more blue.  I have discovered that I have not found any true Blues, Yellows or Reds. 

I always try to follow these “rules”  But I have also found that breaking a rule is fun.  Sometimes it could create a really effective painting.    I have also found that someone may not agree with this process.  I have learned that there are so many opinions out there that finding your own is important.   

The most important rule:  Make time to do your art!