1. use their name and pronouns even if they aren't around
2 Do correct yourself if you slip up
however dont over apologise. simply say sorry correct yourself and move on with the conversation as otherwise it brings alot of attention to it which can be embarassing.
3. Be aware they may not be out to everyone and act accordingly
for instance they may not be out as trans to their parents meaning you may need to use their dead name ect in thoose situations. if you can it may be better to use a gender neutral nickname.
4. go shopping with them
if you are of the same gender of them something that would be really appreciated would be to go shopping with them. clothes shops can be really scary when you are first transitioning and sometimes outright confusing so having someone who understands how it all works would be really helpful. its also a great way to show your support as its going above and beyond.
5. dont comlain about the adjustment
phrases such as 'i'm sorry its just hard' are very blame laden phrases which makes them less likely to ask you for help in the future which could lead to greater levels of anxiety and dysphoria.
6. Look out for signs they are struggling
mental health issues are rife throughout the transgender community with particuarly high levels of anxiety depression and self harm. theese are all problems that the sooner they are helped the more effective the treatment so a really great idea would be to familiarise yourself with the signs and some places they can go for help if they need to.
7. dont misgender them on purpose
getting gendered correctly isnt a privelage so dont misgender them if you are angry ect because its likely that will make them feel even worse
8. use google
your friend may be a great source of info and for some questions you absolutely need to know what they want however imagine how many times they answer questions about their identity a day. for more basic question such as definitions and info about medical procedures ect google works just as well
9. treat them like you would have before
if they were the friend you played computer games at the weekend with still do that with them, if they were the person you explored new places with still do that with them. they are experiencing massive changes in their lives and it can be compforting to have something the same amoungst this
10. offer to go to lgbt groups with them
alot of lgbt groups particuarly in schools/ universities are often open to allies too. in which case offering to go with them could be a great thing to do. meeting other people like them will likely be a great help but particuarly if you are new out it can be scary so by offering to go with them you could relieve some of that fear
bonus tip - if you have boys/girls nights out invite them too. for whats a really simple thing for you to do it will mean alot to them.