The Certificate of Service form most frequently used in cases subject to the Civil Procedure Rules (CPR) can be found here https://formfinder.hmctsformfinder.justice.gov.uk/n215-eng.pdf
In the top right hand corner there are boxes to be filled in with the name of the court, the case number, and the names of the parties.
What documents did you serve?
List in the box the names of the documents served, such as Claim Form, Particulars of Claim, Defence, etc. Many court forms have a form number at the bottom left of the page and if you served a number of forms together and you are short of space in the box you can use the form number to identify a form rather than its name - e.g. a Notes for Defendant form is form number N1C.
Date of ServiceIn the top left there are two dates On what day did you serve? and The date of service is This is because (as explained in the table on the second page) there is a delay between when you, for example, put a document in the post, and when the document is counted as being served.
How did you serve the documents?
Tick the box to indicate how you served the document and complete the address where service was effected. Make sure that you enter the address exactly as it is on the envelope when you post it, if you are using post, or the address where you delivered, if you hand delivered, etc.
Normally once legal proceedings have started, a party will be required to specify - to the other parties and to the court - the address(s) at which they can be served with documents and in this case you would tick other and write e.g. defendant's stated address for service. But when serving a document on the defendant which actually starts off the litigation process (e.g. Claim Form) there are certain valid places for service, depending on the circumstances, specified in the rules, such as the defendant's usual residence, so you tick the appropriate box to indicate which rule you are relying on.
I believe that the facts stated in this certificate are true
The certificate of service should be signed by the person who served the document because it is that person - e.g. the person who put the envelope in the Royal Mail post box (or sent the email, or put the envelope through a house or office letterbox, or personally gave it to a named person, etc.) who knows that the details are true. Normally the person doing the serving, and therefore signing, will be the Claimant or the Defendant so the inappropriate words in brackets underneath the signature should be deleted leaving the appropriate word (Claimant), (Defendant), etc., but, particularly if the documents are being hand-delivered, the Claimant or Defendant might have arranged for someone else, whether a professional process server or a friend, to serve the documents on their behalf, in which case the description of the person signing (e.g. process server engaged by the Claimant) should be stated.
Note: the information above is purely concerned with how to fill in the Certificate of Service form to accurately describe what you have done. Whether what has been done is sufficient to constitute valid service under the rules, in a particular case, is a different question - for example email service may only be valid in quite limited circumstances.