One way is to manually place an apostrophe at the start of every line in the block.
A much easier way (I just found out today) is:
- Go to View-Toolbars-Customise
- Select the Command tab
- Select the Edit Category on the left
- Drag the “Comment Block” and “Uncomment Block” icons onto your toolbar.
Now you can comment (or uncomment) as many lines as you like with a single click.