you should start with this thread on Authorized Digital Sellers [webmasterworld.com]?

My adsense ID is pub-0123435678912345

Customer ID
0123456789

What exactly do I ad to the ads.txt file to make sure my adsense ads show up?

[edited by: phranque at 7:49 am (utc) on Apr 15, 2018]
[edit reason] anonimity [/edit]

All the information you need is in the link phranque gave in the above post.

google.com, pub-******, direct

[fix typo]

[edited by: keyplyr at 7:48 am (utc) on Apr 15, 2018]

did you read the thread i linked to above?
[support.google.com...]

I did, but I see no examples with something after "direct"

google.com, pub-0123456789123456, DIRECT, f08c47fec0942fa0

I think I'm all set now. Thanks.

[edited by: phranque at 11:10 am (utc) on Apr 15, 2018]
[edit reason] for your own sake, please don't post your adsense publisher id [/edit]

Just check the Adsense' documentation : [support.google.com...] it has all the information you need.