Namecheap have great tutorial how to edit DNS record using their DNS manager. Please visit Namecheap help page and scroll the page until you find the TXT Records section.
Add these three TXT records as follows:
| Host | TXT value |
|---|---|
| @ | v=spf1 include:_spf.mailspot.com ~all |
| default._domainkey | v=DKIM1; k=rsa; p=<your public key> |
| _dmarc | v=DMARC1; p=none; |