For this scenario we will use G Suite as the sending service and our domain would be test.ondmarc.com.
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To authorize G Suite to send emails on behalf of our domain we need to create the following SPF record in DNS.

v=spf1 include:_spf.google.com ~all
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Now that we have the value for our SPF record that needs to be created let’s go ahead and add it to the DNS for test.ondmarc.com in Cloudflare.

1. Login to Cloudflare.

2. Select the account under which your domain exists.

3. Select the domain under which you would like to create the SPF record.

4. Click the DNS app as show below.

5. You will be presented with the following UI which is where you will create the SPF record.

6. Create and fill in the fields as shown at the beginning of this article.
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The record type is TXT

The name of the record in this case would be test.ondmarc.com. 

The value portion of the record above contains the following:
​v=spf1 include:_spf.google.com ~all

We will leave the TTL set to Automatic and click Add Record.

Please allow some time for the record to propagate and be detected.