Log into your FastHosts account. (https://www.fasthosts.co.uk/)
Under the Domain menu click Manage Domains.
Select the domain name from the list that you wish to add the DNS records to.
Click into the Configure Advanced DNS link.
A list of DNS records will be shown.
Scroll Down the list and click on the blue icon "ADD TXT RECORD"
Fill out the fields with the following info:
a) Hostname: Enter the host name for the TXT record. For example, enter @ for an SPF record, or enter the value _dmarc for a DMARC record.
b) Destination TXT Value: The value you are setting as the destination of the Host.
When the information are entered, click Save.
Create a free OnDMARC account