Apart from using the Blend tool we just mentioned, you can also brush the model in Paint Weights mode to either add or subtract weights using a set value, or else blend the weights based on the radius as it captures and averages the currently selected bone's influence based on the surrounding area and the other involved bones. The brush settings are accessed via a … button next to the Paint Weights button in the Skin modifier's Weight Properties section.
It helps to set a radius that's not too massive using Max. Size in the Painter Options window. In Display Options, the checkbox Draw Ring shows the radius, Draw Normal shows a line representing the surface normal where the cursor is, and Draw Trace dynamically draws the trajectory of your brush stroke. Normal Scale sets the length of the Draw Normal line. Pressure Options are relevant only if you are using a Wacom tablet or equivalent device, rather than a mouse. By default, pressure has no influence, but you can set...