“While logged into your Twitter account, click the ‘Edit profile’ button on the right-hand side of your profile page. You’ll then be able to change your name, profile and header photos, bio, and more. Once you’ve added a link to your bio, click ‘Save changes’ and admire your work. Simple, effective, and takes minutes to implement.”
It’s so simple to do you’re going to kick yourself for not doing it sooner. The small but valuable real estate below your Twitter profile picture is a great place to link to a subscription form. If you’re tight on space, use a short link to squeeze it in there.