To create a new section 21 or section 8 notice (and equivalent for Wales), first log in to the Builder part of the website, choose "other forms" from the sidebar and click on "add new form" at the top right of the other forms list. Then, with possession notices selected, click continue.
This will take you to the possession notice, wizard.
Once you've gone through the wizard, click the link to be directed to create the relevant notice.
To create a notice, fill in all the fields shown. You can insert from a profile or a previous tenancy agreement, but this is optional.