Just follow this little steps below.
Firstly you will have to sign up for adsense and sign up with your Google account remember that adsense is also own by Google so you don’t need to create any account if you have already.
Now in the adsense page you will be given a form to fill that has to do with your blog and your personal details ao make sure you fill it well without make any mistake. And then submit and wait for review and approval.
Now you will have to create an ad unit then copy the code and then visit your tumblr dashboard just click the title at the top.
On that same page check on your left hand side of the page you will find customize theme just click on it.
Now on the then page you will have to edit the html code by clicking on edit html and then insert your adsense code either to your footer,body or header.
Finally just hit the save button and then check your blog.
Don’t forget to drop a comment below thanks.