How do I implement Ord for a struct?

The definition of Ord is this:

pub trait Ord: Eq + PartialOrd<Self> {
    fn cmp(&self, other: &Self) -> Ordering;
}

Any type that implements Ord must also implement Eq and PartialOrd<Self>. You must implement these traits for SomeNum.

Incidentally, your implementation looks like being the wrong way round; if self.value is all you are comparing, self.value > other.value should be Greater, not Less.

You can use the Ord implementation on u32 to assist, should you desire it: self.value.cmp(other.value).

You should also take into account that Ord is a total ordering. If your PartialEq implementation, for example, takes name into consideration, your Ord implementation must also. It might be well to use a tuple for convenience (indicating that the most important field in the comparison is value, but that if they are the same, name should be taken into account), something like this:

struct SomeNum {
    name: String,
    value: u32,
}

impl Ord for SomeNum {
    fn cmp(&self, other: &Self) -> Ordering {
        (self.value, &self.name).cmp(&(other.value, &other.name))
    }
}

impl PartialOrd for SomeNum {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl PartialEq for SomeNum {
    fn eq(&self, other: &Self) -> bool {
        (self.value, &self.name) == (other.value, &other.name)
    }
}

impl Eq for SomeNum { }

If you’re doing it like this, you might as well reorder the fields and use #[derive]:

#[derive(PartialEq, Eq, PartialOrd, Ord)]
struct SomeNum {
    value: u32,
    name: String,
}

This will expand to basically the same thing.