Do you remember when we did loads of cassette duplication of news for blind folk - posted out each week, then returned. We had bulk erasers for these, and different ones for reel to reel tapes. I think the one I had was made by Weircliffe -
<img src="https://thumbs.worthpoint.com/zoom/images1/360/1112/04/weircliffe-bulk-eraser-model-vintage_360_a6f49ab599ce2dc965094ef0fc55cabb.jpg" alt="WEIRCLIFFE BULK ERASER MODEL 8 / VINTAGE / EXCELLENT CONDITION (image 1/4)" /> In a big wooden box. you posted the tape into it, and pressed a button. A huge magnetic field scrambled the recording and then it slowly died away to nothing. I remember mine worked fine on the reel to reel tapes I had in the 70s. It couldn't erase Chrome Dioxide tapes though - it always left a trace of the old recording.
If you want to just demagnetise a head you can get one for twenty five quid? In all the years I as a teenager used them, the only problem was just making sure you didn't touch the heads - and most of the ones I used had a plastic sleeve so that even if you did, accidentally, you didn't damage them.
The only thing to know in advance is that you must power down the machine, because the 50Hz blast of hum through the speakers can be quite nasty, but the worst thing, that nobody mentions is that the things vibrate when they are in the vicinity of the sort of metals that they work on - you can feel the magnetism working. A strange sensation.
All I know is that when you make a vital reel to reel recording, or a cassette recording - then you clean the heads and tape path and you demagnetise. It is like checking tyre pressures. You know you should, but usually don't. It is just good practice, and a good habit. It really is no big deal to do. Place close, turn on, move away across the room, switch off. That really is it.