To use the signature, you have to find the Certificates tool (click down arrow to add as shortcut to right bar of tools). It is NOT the Fill & Sign tool:
Click Tools at the top left of the page:
Then look for:
When you click Open, the following menu opens at the top of the document:
Click ‘Digitally Sign’. This opens the draw box dialog box.
Draw a box.
The Sign with a Digital ID dialog box will open. People usually have only one choice. Click 'continue'.
If you want to use the signature set up in step 1, click the Appearance dropdown to show your list of signature appearances:
Because locking will often hold up the next step, the recommendation is to leave this box unchecked. The document can be protected by printing to PDF after the signature process is complete.
Then click ‘sign’.