From FreeCAD Documentation
- Activate the Body which is to receive the Boolean feature. Note: It is important that neither the active Body nor any of the features it contains be selected!
- Press the PartDesign Boolean button.
- In Boolean Parameters, click on the Add body button. The active body temporarily disappears from the 3D view to facilitate tool body selection.
- In the 3D view, select the Body to use in the Boolean feature. Repeat to add more Bodies.
- Select the type of Boolean operation in the drop down menu (Fuse, Cut or Common)
- Click OK.