Hold the tool by the handle, but avoid grasping the metal ferrule if there is one. If the ferrule becomes loose, it could pinch the skin on your hand as you grip the handle.
The chase wedge should be held firmly to secure it in your hand. However, avoid gripping it so tightly, that your knuckles turn white!
Use a chase wedge
To assist with bossing sheet metal
A chase wedge is used to assist with bossing lead work or other soft metals into different shapes. When setting the work out, the chase wedge is placed in contact with the lead work and is struck with a mallet to set in the fold lines and angles, for example.
The main aim of bossing is to achieve the required shape without making the lead too thin or too thick. To avoid an uneven surface or a fracture, care should be taken to avoid stretching the lead.
To fit lead work into position
When the lead work is complete, the chase wedge helps to dress it into position.
For instance, place the edge of the tool against a piece of new flashing around a skylight or a chimney prior to pointing (filling the chase with mortar). Then apply a few gentle blows with a mallet. This will secure the flashing into any difficult configurations or awkward angles.
Sheet metal bossing is not a skill you can learn just by reading. Take the opportunity to observe a skilled lead worker, then practice the techniques yourself.