Bring mouse in any viewport, move the cursor over the object you want to select. The cursor changes to a small cross when it’s positioned over an object that can be selected.

While the cursor displays the selection cross, click to select the first object.

Now hold down CTRL button on the keyboard, and click on the second object. First and second both objects will be selected.

If we click on the third object, holding CTRL button, the third object is added to the selection.

Similarly click on fourth, fifth, sixth, seventh objects (and so on), holding CTRL button. All these objects are added to the selection.

Release CTRL button.

Click an empty area anywhere outside the current selection.

Hold down the ALT key, and either click an object, or drag a region around the object to deselect it.

Hold down the CTRL key and click to deselect a selected object. This also selects non-selected objects.

Choose Edit menu > Select None to deselect all objects.