Painting Portraits in oil on canvas
Once you've decided on your subject, it's important before to start your portrait, in your minds eye, work out the composition, and the geometric perspective within your design, if you're good at drawing you can sketch this roughly in pencil on your canvas, however if you're not good at drawing, it might be helpful to start by using a grid system, and if you are working from a photograph it's best to photocopy your chosen image so you don't spoil your original; then draw your gridlines on your photograph copy, this will give you an accuate way to copy your design onto your canvas which you've already drawn your grid lines, scaling it up from the smaller photocopy image.
Once this process is completed, I prefer to use fast drying oil based paints, a good make to use is Winsor & Newton, Griffin ALKYD. They're fabulous to achieve smooth skin tones. They're also good in that you will have to build your design up in layers, starting off in monochrome. It's important to let each layer of this process to completely dry before painting your next layer, this is why fast drying paints are good to use, otherwise you could be waiting ages inbtween each different layer to dry when all you want to do is carry on painting your design. Don't worry if you make a mistake when painting, just wait for it to dry before painting over it to correct it.
You do require a little artistic flair when you build up your painting in stages, be careful how you mix your colours, look precisely at the area in your photograph at the region of colours you are working on and mix these colours accordingly. Mainly you just require a keen eye for detail. This is not a free flowing artistic process, it's more about copying your design exactly how it is. It's good practice when you are learning, once you possess the ability to paint an object to look like a photograph then it's time to experiment with your natural artistic Impressionistic styles, sometimes you'll be surprised at the result, sometimes it may not work out but occasionally you'll find you create a master piece, they call it in the art world a happy accident.