I’m wondering what would be the best way to go about this. I have family and friends who I can no longer associate with because they lead bad lives and are occasions of sin. I haven’t told them yet but I will have to soon. 

With family, it can be tricky because they can live close or you depend on them. I would just put some boundaries and busy yourself so you can tell them you’re too busy to socialize with them. They’ll get the idea and stop initiating, I hope.

Can confirm this doesn't work for some.

Telling family and friends you can no longer associate with them

Why do think you need to tell them? Simply ignore them. They're not your owner. You have no obligation to be beholden to scandalous family and "friends".

Why do think you need to tell them? Simply ignore them. They're not your owner. You have no obligation to be beholden to scandalous family and "friends".

I guess I should explain some. 

I’m a convert so I used to be close with family who are living worldly and scandalous lives. They have no concept of avoiding sin or what it means to live a virtuous life and I haven’t really “told them” too much, so some of them still think that they can just show up to my house for social visits, for example. 

I haven’t “made them aware” that I won’t be like I used to be or hang around with them anymore so it’s this first instance that I’m not sure how to go about without making it unnecessarily bad. 

Also, I moved out of the country and don’t live close to them anymore so they will have to fly here to come visit. They may or may not let me know in advance, so I don’t know if I should preemptively tell them so they don’t waste their money since tickets are usually non-refundable. 

Relatives are going to want to visit you and your child.  You need to have some adult conversations and explain to your family the conditions for them to be in your new/faith-based life.  Your motivation in all of this is charity.  Your motivation is their understanding of your new life and, by your humility in explaining it, then hopefully they might convert sometime in the future.

Writing people off, with no warning, is not charitable, or humble, or mature.  The Faith is not spread by isolation.  If you explain your new faith in a humble way, you might be surprised at the changes people make to be agreeable to your Christian lifestyle.

It won't be easy but you have to try.  Then, if you get negative or aggressive feedback, you can cut them off.  And it won't be your fault, it will be theirs.

Yes, that is exactly what I want to avoid. Thank you for the advice. 

OP here.

There’s really no option because I have a child now and I know it’s a parent’s duty before God to prevent your children from being corrupted by anyone, even a family member, and this is what would happen if I were to let certain people come over to visit, which I know they will want to sooner or later.

What makes the situation a lot worse now is the existence of smartphones because they can show anything or get in a video call with anyone in an instant.

I won't give advice on how to do it, but do it - not only for yourself, but for your children.

In the 60s, Deo Gratias my parents did it, and it is one concern that I have never had. All of our relatives went NO when the changes were happening, and my parents had a more than a few big arguments and battles over it with relatives and friends, but it didn't take long for them to simply stop showing up and never come back. None of them even showed up for my parent's funerals. To this day I have never had to deal with any of that - thanks to my mom and dad doing what needed to be done when I was just a kid!   

Also, I moved out of the country and don’t live close to them anymore so they will have to fly here to come visit. They may or may not let me know in advance, so I don’t know if I should preemptively tell them so they don’t waste their money since tickets are usually non-refundable.

First let me just say I would dislike anyone just dropping by unannounced. Unfortunately, you've allowed that to happen all of this time. 

Having said that, would they really fly out to another country and just show up at your door? That seems a bit much even for the most obnoxious and inconsiderate of persons. 

I don't think you need to do anything given the distance.