Hello, anon! I’m flattered that you think I’m someone you can come to for this situation. I’ll be doing a step-by-step format. 

• Since I do not know you or the kind of relationship you have with this particular friend, you need to pick a place to tell them where you are both comfortable but able to hold a semi-serious conversation (preferably somewhere private)
  
• Let them know that what you are going to tell them is something important to you and that you wish to share with them. The manner in which you say this can be casual or serious (again it depends on your relationship with them). Ex: “Hey, I just wanted to talk to you about something serious” or “I have something important to tell you.”
  
• Tell them that you struggle with/have anxiety. This is probably the toughest part because it can be such an emotional thing to let someone know depending on your feelings and personal experience with it. This is why a comfortable (and private) environment is important. Maybe say something along the lines of “I just wanted to tell you that I have anxiety,” Maybe personalize it a bit, depending on how much you want to talk about it. I’ll include a proper example at the end. 
  

• Tell them the situations that trigger your anxiety or make you uncomfortable. If you’re scared of overwhelming them too much maybe start out with the ones that have come out more often in their presence or one’s you think may come up.
  
• Let them know whether or not you have anxiety attacks often (or how often you have them if at all). Also let them know the symptoms of your anxiety attacks (since it can differ from person to person) and what can help you calm down in those cases. 
  
• Let them know whether you care if they tell anyone about your anxiety in case anything that triggers your anxiety arises in the company of others. 
  
• Finally, don’t let them make you feel like this is your fault or something you can always control. In terms of friends, this has never been an issue for me. Unfortunately, I feel like it’s usually something that older adults or doctors make one feel. I’ve been told countless times by relatives and doctors that this is just something I have to control and that I’m not trying enough to “get better,” but anxiety is a complex thing. 
  
• Thank them for listening and being understand (if they were). 

My anxiety started developing around middle school or high school and is also enhanced due to a medical issue I was recently diagnosed with (oddly enough, it seemed to calm me down knowing there was some partial explanation for it). 

I recently (once more) told someone about my history with anxiety (since it has been getting better recently and has made me slightly more willing to do things I would not have done before). Their reaction to it was surprising (in a good way) and made me so happy that I told them. And I really hope that your friend makes you feel the same way. 

Example:

I have anxiety and although it has been getting better lately, it’s something I still struggle with it. I’ve been thankful enough to have a supportive group of friends that have helped me with this. But certain situations still tend to trigger my anxiety. I don’t usually like it when people hug or grab me without my permission or being in environments like parties where it is expected of me to be very social. I tend to be wary of strangers, and it takes me time to open up to people. Comments on why weight trigger me because of past experiences. Sometimes I tend to work myself up to the point that I end up having an anxiety attack and just need time to calm myself down. 

Hopefully this helped you. I hope everything goes well with your friend, anon, and feel free to message me about this anytime.