Download the official Ullu app from the Play Store or App Store. Look for the "Free" tab or "Watch Free" banners on specific series.
Ullu maintains an active YouTube channel where they post trailers, "best of" compilations, and occasionally full-length clips. While you won't find the entire 18+ library there due to YouTube’s community guidelines, it is a great way to see highlights and decide if a particular series is worth a subscription. Why You Should Avoid "Free" Piracy Sites Watch Ullu Web Series 18 Video For Free
The best way to watch for free is directly through the source. Ullu often makes the (and sometimes the first few) of their popular series available for free to everyone. Download the official Ullu app from the Play