Method 1:

In your blogger account, click on Monetize, under the blog you want to add Adsense to.

Now select whether you want to display ads in posts, sidebar or both. You can also choose to not auto-display ads and add your code manually. Or you can choose this option if you have already enabled ads and want to use the blogger interface to only monitor earnings.

When done, click NEXT. In case you have not furnished your adsense account details, you will be able to do that, otherwise you will see the following screen.

Your adsense is set up. From the above screen, you can track your earnings, customize the locations of your ads, or make account changes.

Method 2:
In your blogger account, see first figure above, click on Layout, under the blog you want to add Adsense to. On the following screen, click on Add a Gadget in the sidebar or the footer.

Choose Adsense on the next screen.

Configure the ad size, color combination, etc., hit SAVE and you are done.

For adding more ads, repeat the same process. Just remember, you can only add up to three ad units and three link units on any page. You can also move the ad widgets to wherever you want – between the sidebar, header and the footer.

Track Earnings Using URL Channels
If you want to add the code on your own, choose HTML/Javascript under Add a Gadget. Managing your own code is more work, but it gives you more control over the location and colors of your Adsense, and also helps you track the earnings using Channels. At present, there is no way of adding channels to Adsense using the automatic Blogger Adsense feature. What you can do, however, is use URL channels to monitor the earnings on blogs you add code by the easy method.