You've learned how to tune and hold your ukulele, and you know one chord. Before you set out on your first strumming exercise, let me show you what strumming looks like. This wee video was shot from over my shoulder, so you can see how it's supposed to look, so when you look at what you're doing - you'll have something to compare it to.
Nothing fancy, but it can be difficult to picture. The nail of your index finger faces down - and it is your fingernail which strums the strings on the downstroke. And unlike a guitar, you don't strum near the sound hole - you strum further up, near where the neck joins with the body of the ukulele. No matter how many times we describe it with words - it can really help to see it, and see it from this perspective.
On the next page, you're going to "strum along" with an incredibly simple musical passage. No chord chages yet - just practicing the sonic, rhythmic component all by itself.