Click on Variables in the Code tab, then click on Make a Variable.
Type in the name of your variable. You can choose whether you would like your variable to be available to all sprites, or to only this sprite. Press OK.
Once you have created the variable, it will be displayed on the Stage, or you can untick the variable in the Scripts tab to hide it.

Add this line inside the forever loop of the pencil sprite’s code:

The pen width now repeatedly gets set to the value of the width variable.

Right-click on the width variable displayed on the Stage, and then click on slider.

You can now drag the slider that is visible below the variable to change the variable’s value.

Test your project and see if you can change the pen width.