Here is a step by step procedure :
1.GO to DISQUS.
2.Click on Sign Up,Free
3.Fill The details on the Sign up page.(click on the image to enlarge) and Click on Continue
4.Fill the settings Page(You can skip this step and change your settings later.)
6.Choose youe blogging platform
7.open Blogger.com in new Tab
Login to your blogger dashboard.
Layout >> Edit HTML>> Download Full template
8.Revert back to Disqus tab.
CLICK on Choose file .
A new window will open Chosse the select xml file which you have downloaded in step 7.
If u want to import previous comments from blogger Click on 2nd check box...like this
Use Disqus Comments on all blog entries. Any existing comments can be imported in.
Click on Upload and continue
9.COPY THE XML code given by the Disqus.
10.Revert back again to Blogger tab.(Edit html tab)
11.Click on Expand widget template
Expand widget template
12.Paste the code(copied from disqus site) in to the edit tempate area.
13.Click Save Template
14.View blog to see a new DISQUS comment system on your blog.
Whats more ???
You can use Disqus to import your previous blogger comments .
You can import comments from .other commenting system like intensee debate
You can add comment moderators to your account.
You can customize your comment system with every color combination.
Use disqus widgets for blogger.