Draw features. Once you're done, choose an action -- it will send a request to the server, and redraw the features.
If you'd like, you can just select a few features, and click the processing button -- if features are selected, they will be processed rather than the whole layer.
As always, you can hold down shift to draw 'freehand', in sketch mode. This is especially useful to see the 'simplify' functionality in action. To stop drawing in freehand mode, just let go of the mouse -- in non-freehand mode, double click to stop.
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