The letters tell you where to put the numbers.
Consider the following expression:
$$9b + 8d$$
If we are told that b = 9 and d = 0, then we can put the numbers in, like this:
$$9(9) + 8(0)$$
As a general rule, you should always put brackets around numbers that have been substituted in.
Sometimes they have an effect, sometimes they don’t. But it’s always useful, not least because they show us which numbers have just come in.
And that’s it. That’s all that substitution is.
In practice there will be other things you need to do after substituting, but we’ll discuss that elsewhere.