There is an excellent book called Ukulele Fretboard Roadmaps - The Essential Patterns That All the Pros Know and Use that is an essential book if you're wanting to learn everything about your ukulele fretboard, including movable chords (barre chords), etc.
The YouTube video that I'd recommend first and foremost is the Stuart Fuchs video immediately below. I attended one of Stu's workshops at Midland Ukulele Festival in which he explained his concept of the "heavy hand" and using gravity, which is invaluable, not only for barre chords, but to enhance your ukulele playing. You'll find your chord transitions so much better and more beautiful when you follow his advice. I've included some other Stu Fuchs videos about barre chords as well. I've also posted a Bernadette Teaches Music video which is also good.