I was surfing through a forum few days ago and a guy asked how to place ad codes on blogger's blog. Then I decided to write on this.
For you to place any ad code on your blog, You must have gotten your ad code.
Kindly follow the steps below:

Firstly, Login to your blogger dashboard as you can see in the image below... In The Blogger Dashboard scroll down to LAYOUT and click on it as seen in the image below.

A new page will appear as seen below. Just surf around the new page a find a suitable bar to place your ad code...

But i advice you place it in the sidebar and below title navigation bar so it will be visible to all visitors in your blog.
When you are done click on ADD A GADGET button.

A new page will show as seen below then click HTML/JAVASCRIPT.

A page will pop up then you will see a box/textfield to post in the Ad Code... Finally, Click on save button...

Check Your Blog to see the ads displaying... Have A Nice Day.

Source:
http://www.gidimasters.com/2016/02/how-to-place-ad-code-on-your-blog.html