Install Krita from the official project. It is free open-source painting software; optional store editions can cost money. Create a modest canvas, for example 1600 × 1200 pixels.
Choose a brush, select a colour and make a few strokes. Put your sketch on one layer and colour on another. Test pressure sensitivity if your tablet supports it; mouse drawing also works.
Save your editable work as .kra, then export a PNG copy. Use Undo for mistakes and keep periodic backups. Exporting an image does not preserve every editable layer or feature.
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