How to set up SPF for Google Apps and Sendgrid

I’m using sendgrid.com to send emails from my Django app to confirm user’s emails during registration, and for other notification purposes. The sender for these emails is webmaster@ifbt.farm. I added CNAME records per the sendgrid instructions.

I later set up a Google business account with two email addresses, ddrake@ifbt.farm and kdrake@ifbt.farm. I added MX records per the Google instructions.

I’ve read a bit about SPF, and I understand that I need to add a txt record, with an include directive, but I need to be sure that I’m not blocking any of these emails.

The article https://www.digitalocean.com/community/tutorials/how-to-create-a-spf-record-for-your-domain-with-google-apps seems quite old, and contains a broken link.

Can someone help me out with the correct SPF text line for this case?

Thanks