If you are a UK national, you should contact the following address of the passport office which is located in London square:
Tourist, business and transit visas are issued by them.
Following people don’t need to apply for a visa:
a) People having multiple entry visitor's visa for UK
b) Permanent residents (ILR)
c) Those with a certificate of entitlement to the right of abode.
But the following people do need to apply for a visa:
a) People who hold only 6-months visitor visa
c) Work-permit visa holders living in UK.
In case you belong to one of the above and are currently in London or some other place in UK, you need to contact the relevant embassy to get a tourist visa for visiting Guernsey.
Country travel advice: See the following travel guide "Link" for potential threats and cultural awareness occurring recently in your desired destination.