If you want to add the Customerly SPF record on top of the SPF you are already using, follow the instruction below:
Let's assume you have google SPF in your TXT record:
v=spf include:_spf.google.com ~all
What you need to do is adding the Customerly SPF in this way:
v=spf1 include:_spf.google.com include:spf.customerly.io ~all
Just append the include:spf.customerly.io after your current SPF and before the ~all
Let us know if you have any questions. We are happy to assist you!