‘Every non-trans person can be an ally for trans equality, but not
everyone feels confident doing this. It’s not complicated’.

This is from the March 2018 guidance (PDF) from UNISON on supporting trans people in the workplace.

The guidance recommends:

Speak up when there are no trans people present. Transphobia is always wrong and shouldn’t be ignored
Misgendering someone or using their previous name (sometimes called ‘deadnaming’) is hurtful and may be unlawful harassment
Try not to make assumptions. Although there aren’t many trans people (which is why allies are so important) they are all individual. There is no one way of being trans.
Read the guidance (pdf).