To use this feature, it's important that you either use Blogger in draft or by enabling it via the settings tab:
So now, when you go to write/edit a blog post, you'll see you have a more updated editor bar:
And you'll see in the above grab I'm pointing to the "jump" function. Now I'm not going to give you a massive spiel of instructions on how to use this but if you want to know more or if this is all new to you then go here to read the official Blogger info. It's quite useful.
BUT.... if you're using a modified template then you're going to have to edit your html code to ensure this functionality shows up.
To do so:
1) Go to Layout > edit HTML tab in Blogger
2) Click Download Full Template (you should always do this in case something goes weird and you want to re-install your template). OK, now we can get down to business:
3) Click on the "Expand Widget Templates" button. OK, now, down to business: click the Expand Widget Templates checkbox, and scan your HTML for the following snippet:
Once you've located that code, paste the following snippet directly below it:
a expr:href='data:post.url + "#more"'> <
Then click SAVE and you're done.
Whilst I did test this and it works, I have no idea how to fix any issues regarding this, so please go to the Blogger help forums if you're having trouble.