how to make nsfw age verify popup block thingamajig on my own site? (NOT for fr don't worry)

when i tried googling this i just got downloadable browser extensions to block all adult content so kids/old people didn't stumble on it by accident which is not what i want

backstory: i am going to start a site where i regularly post short stories of my characters. however the characters are adults and sometimes i want to portray or discuss adult topics. 

for my stories with nsfw and/or potentially triggering content in them (transphobia experiences for example) i would like there to be a popup on the page verifying that the user is willing to see that kind of content.

YES I KNOW KIDS CLICK THROUGH ANYWAY but this is also for peace of mind for someone who might not want to read certain things at certain times. it's a courtesy. 

how do i create such a popup for this and what is the best way to go about it? since i'm not collecting data it doesn't need to have a place to enter a birthdate either.

thx!!!!!

If you have accessibility requirements you probably want to use a framework like Bootstrap and a modal pop-up. If you don't care too much about disabilities you can probably use CSS and a bit of Javascript to make a modal pop-up.