I shouldn't scold, but you really should have mentioned 2007 the first time.
It's different enough that it needs a different set of directions to get
started.
First go to Office button > Word Options > Popular and check the box to show
the Developer tab in the ribbon.
Now that you can see the Developer tab, look in the Controls group. There's
an icon that shows a folder with a couple of tools, and a downward-pointing
arrow. Click that to open the Legacy Tools box. The top row, labeled Legacy
Forms, has the same icons that were on the Forms toolbar in previous
versions. Use those to build your form (and don't be tempted by the ActiveX
controls in the bottom part of the box).
The one item from the old Forms toolbar that isn't on that box is the "lock"
icon for protecting the form. Instead, the Protect Document button is
further to the right on the Developer ribbon, and it opens a task pane where
you can choose to protect for filling forms (among other kinds of
protection). That protection is what makes the form fillable.
After you know where the buttons are, the rest of the procedure for building
and using a form is the same as described in the article.