A post-it note works if you don't have a slider.
The Electronic Frontier Foundation sells a set of stickers exactly for this purpose [0]. I have a set and it works reasonably well. And it supports a good cause.
[0] https://shop.eff.org/products/laptop-camera-cover-set-ii
People won't look at you the same way after you do this. And it won't be for the better
It's an extremely common practice in enterprise IT, literally nobody gives a shit.
More effective than trying to get permanent marker ink to block light getting through the lens.