First you have to establish common ground what the boundaries of their comfort level are. For most people God represents the principle of goodness so you don't want to take an approach that makes you look like you're coming in the opposite spirit. I like asking moral questions and pointing out conflicts between the bible and even their own morality. Like, is the person pro-slavery, pro-genocide, etc. What you shouldn't focus on trying to prove is that God doesn't exist. Instead, the main focus should be that humans have shown, all around the world, an extreme willingness to attribute their own words and explanations as if it came from God. But meanwhile we can see that it was about their own wealth and power.

Everyone must make that journey & arrive at a destination that may change thru the years. Faith is merely believing something with no evidence. One must decide for oneself whether science, facts, evidence, matter more than fantasy, doctrine, tradition. Because one believes does not make it so. Tolerance, patience & security are what you can provide.

Smile, nod, listen and let them talk their way through it. If they've already started down that path they have a reason. Let them talk it through without trying to put your own input into their transition.

It lasts longer if it's an epiphany that they have using their own reasoning than if someone feeds the arguments to change their mind.

Change the subject.

Carefully if they asked first

Reading. To combat creationism read Neil Tyson, Carl Sagan, etc.

To combat the mythology have them read some Greek mythology, with an open mind the concepts for the Christian God that were stolen from Zeus are obvious. Jesus = Very Weak Heracles.

History. Ancient history and philosophy. Natural history. Let them see its scientifically proven that the earth is a hell of a lot older than 5k years.

Also one of my favorite tools. All 3 Cosmos series. Sagan then the two with Tyson. The "cosmic calendar" used by Tyson in his two seasons is mind opening.

Avoid things like YouTube (by and large inaccurate and lacking in any and all real information. Remember YouTube is where qanon got its start) and random websites which are far from legitimate.

Encourage them to learn some history, and how it works, for example recommend a secular history of the biblical lands. Does not have to be a book if they don't read, a Youtube documentary is a good start. There is nothing destroys religion like knowledge.

I always tell people that our thoughts and believes are the only true possessions we have and the only things we alone have complete control over. You do not need to agree with me but if you want me to respect your thoughts and believes I need to believe that you truly respect mine. We may feel more comfortable around those who share in our beliefs but we only grow by listening to and respecting those who posses thoughts different from our own.

I would say nothing until asked after your friends or relatives ask questions about changes in your behavior . We are not the people who preach and try to convert. I suspect that you have thought through the way to your current position and they have to do work too. A good friend or relative would also do some research before they criticize you.

I think the starting point is learning how to distinguish fact from fiction, namely understanding what exactly constitutes credible evidence. If they take that journey truthfully, everything else falls into place. Important question for discussion.