Cutting-in is approached like this:
Place a loaded brush just beside an edge, then apply slight pressure until the bristles spread up to the edge. Draw the brush down without increasing the pressure and watch your line. Brush in a slow, continuous motion. When the paint begins to run out, reload and carry on, but begin again a metre away from your last brush stroke and work backwards to join up to it.
Painting an external angles, such as the top edge of a chest of drawers, avoid brushing inwards over the angle as this will act as a scraper and create a splodge of paint which then drips down the vertical surface.