Approach the corner with room to breathe
Work the straight line until the corner is close, then reduce the number of stitches loaded on the needle. A shorter group gives you more control over the turn and makes it easier to keep the corner aligned with the marked design.
Avoid arriving with a long, tight thread pulled across the last few stitches. Smooth the cloth before changing direction.
Turn the line rather than forcing the cloth
Follow the marked corner with the needle, leaving a small amount of ease in the thread as it changes direction. The thread should lie on the surface, not act like a drawstring pulling both sides together.
On a sharp geometric corner, a tiny adjustment to stitch length is less distracting than a puckered point. Aim for an even visual rhythm rather than a mechanical measurement that damages the fabric.
Inspect the reverse after a few turns
Turn the piece over after the first pattern repeat. Check for crossed jumps, bulky knots and thread that has caught on the corner instead of passing through the intended layers.
If the corner draws in, unpick only to the last flat section and try again with fewer stitches, a lighter pull or a more suitable needle.