First, of course we have to open the site and the video
If your Internet connection speed isn't very fast, you'll have to wait until the video appears. Then it will look like this:
Just ignore the advertisement, you can close it by clicking the x sign. Now scroll down a bit, and you will find a button under the video. I marked it with red circle ^^
Click it, then another tab will open like this:
What I marked with red circle is a HTML code to embed the video. Just copy and paste it on you blog and the video will appears ^^
While what I underlined with yellow line is a code to make this:
And I have to note that not all blog service could embed video from Nico Nico Douga, so if the HTML code didn't work, maybe your blog don't allowed embed video from Nico Nico.
That's all from me, sorry for my bad English. I just hope this will help ^^