Open Krita
Select the brush tool
Choose a brush preset to use as a starting point
Open the Brush Editor
Adjust brush tip, size, opacity, spacing, and other settings
Test the brush on the canvas
Refine the settings until satisfied
Open the Brush Presets docker
Click the save new brush preset option
Enter a name for the brush
Choose a tag or category if needed
Save the brush preset
Use the new brush from the brush preset list