When taking on a volunteer, it’s a good idea to formalise and record your process. You can do this by using a Volunteer Role Description to advertise the role and a Volunteer agreement to clarify the arrangement when you find somebody. This can help you to avoid any disputes, for instance, if a volunteer argues that they have an employment relationship with you.
Sometimes, you also need to perform checks when taking on a volunteer. For example:
immigration checks, if you’re taking on a volunteer who requires a visa to volunteer in the UK
criminal record checks (eg DBS checks),  if your volunteer will be working with vulnerable people or similar
For more information, read the Government guidance.
You should also always adhere to equal opportunities and discrimination laws when taking on volunteers. Having an Equal opportunities policy in place can help you to do this.