at once using a table. This is how I do it.
In this example I have five list items already in a document.

Below the list, I will add a 2 x 5 table.
Select and copy the list into the second column of the table.
In column 1, row 1, insert the check box as described in the article
copy the check box
select the remaining empty cells in column 1 and paste the check box.
All items now have a check box next to them.
Go back to your original list and delete it.

You can now format the table to not look like a table. To do this
select the table
right mouse and select table properties
from the table tab, select borders and shading
from the borders tab, select none.
press ok twice.

The whole process takes less than a minute, no matter how large the table.