Note that the following steps are described in this page of the Ubuntu Packaging Guide.

You first need to send your PGP key to the Ubuntu keyserver. To do this, first run gpg --list-keys to find out your key ID. The key ID will be the part after 2048R/ or 4096R/. Next, run gpg --send-keys --keyserver hkp://keyserver.ubuntu.com keyid; replace keyid with your key ID.

For the second part, run gpg --fingerprint to get the fingerprint of your key. This part is in 8 blocks of 4 characters. Copy that, and paste that in the text box in the page in the Edit PGP Keys link.

After submitting, you should see that a key was added, and you should get an email saying that a PGP key was added.

Note that you also need to create an SSH key. You can run ssh-keygen -t rsa to create a 2048-bit SSH key. It is recommended that you have a passphrase for this key, as this key can not only be used for communication between your computer and Launchpad, but could also be used (by you) to access your computer remotely. Run cat ~/.ssh/id_rsa.pub, and copy the output. Then, paste this into the section on adding an SSH key.