UNDERSTAND THE TECHNIQUE
Use the long corner as a guide
The angled brush head ends diagonally. One corner is longer. Depending on your grip, you can place it down first. Position it deliberately at the desired point, then bring the rest of the edge into contact. This sequence helps in an inside corner or along a narrow boundary, for example. Hold the handle so you can see the starting point. If your hand hides the important area, turn the paper or adjust your sitting position before painting.
A straight edge comes from guidance
The angled shape makes contact easier, but does not draw a straight line by itself. Plan the path and move your hand in one steady stroke. For a longer boundary, bring your forearm into the movement; your fingers do not have to cover the entire distance. Practise short edges first. Extend them only once the pressure stays consistent. A lightly loaded head is often easier to control than one with a visible drop of paint on the long corner.
Switch between a broad area and a fine mark
Contact across the broader side creates a wider stroke. Using only the long corner with light pressure makes a finer mark. A gentle rotation can connect the two effects. This is useful for leaves, small ornaments and decorative lines. While practising, watch when the short corner starts making a mark too. This can be intentional, or it can leave an extra line. First change only the brush position, keeping paint and pressure as consistent as possible.
Distinguish wet and dry layers
A clear boundary is easier when the underlying layer does not unintentionally absorb or merge with the new application. With acrylic, you can deliberately paint over a dry area. With oil and other media, the sequence depends on the technique. If you want a soft transition, paint that is still workable can be helpful instead. Check the area before applying colour and decide what kind of edge you really want.
Correct common problems precisely
A wavy edge often comes from an awkward movement. If a thick spot appears at the start, load less paint. An extra mark may come from a corner that is also touching the surface. Do not try to solve every problem with more pressure: this usually increases the contact area. Clean the head if needed, realign it and test on a spare piece. A liner remains a useful alternative for a very long, hair-fine line.
All sketches show shape and movement direction schematically. The actual paint mark also depends on paint, surface and moisture.