How do I make an invitation letter to invite someone to the country?

My girlfriend and I are doing all the last bits of paperwork in preparation for her flight back to Spain.

She does not need a visa to come here, due to political agreements between our countries, but she does need an invitation letter to show at the airport.

The problem is, the website I have found, the official site, does not seem to specify what an invitation letter should contain. I imagine I will need to add my details, but don’t really know exactly what this is.

Can anyone help and inform me?

Thanks