To start, choose whatever colour you want! Paint a strip along the bottom of your nail, try to make it cover less than half of your nail. Do two coats if one isn't enough to give a good block of colour.
Don't worry about making a straight neat line, because you will be blending the line with the clear varnish anyway. As you can see mine are pretty messy.
Wait a minute or two for the colour varnish to dry a bit, but not completely. Now for the blending, use the clear varnish brush and poke it toward your cuticle along the line of colour, this will soften the colour varnish and tidy up any messy lines.
Once you have softened the edge of each colour line, coat the whole nail in clear varnish, dragging the blended edge of the colour varnish upwards.
The end product should look something a bit like this:
But perhaps a little tidier and nicer than mine, I'm not the neatest person I know, I prefer how they look after a day of washing and wear, when the messy edges of nail varnish have washed off a bit. Anyway, hope that was fun or useful to someone.