If you're completely new to Flow®, simply go to the Flow® Mixing Suite page and click "Start My 14-day Free Trial". You'll then get to choose whether you'd like to pay for Flow® Mixing Suite, Flow® Mastering Suite, or Flow® Complete Suite (after the free 14-day trial ends).
No matter what subscription plan you choose at this stage, you'll always get the Flow® Complete Suite during the trial period. After the trial period ends, you'll be charged either 14.99 USD/EUR (for Flow® Mastering Suite or Flow® Mixing Suite) per month (and earn 30 credits/month), or 24.99 USD/EUR per month (for Flow® Complete Suite) and earn 50 credits/month).
If you're a current Flow® Mastering Suite subscriber, you can still demo Flow® Mixing Suite for free, but from your "My Subscriptions" page. This demo is a regular 20-day demo, and you will not automatically be charged when it expires (instead, you can upgrade to Flow® Complete Suite or sidegrade to Flow® Mixing Suite from your "My Subscriptions" page).