Go to Dashboard/Profile (avatar, bottom of the left rail)/Profile…/Plan/Upgrade to Pro — or click "Upgrade to Pro" on any Pro-locked feature elsewhere in the app. Choose monthly ($6.98/mo) or yearly ($69.98/yr) and complete checkout — Pro features unlock as soon as payment goes through.
If you started checkout before creating an account, finish signing up first; checkout automatically resumes once you’re signed in.