I was terrified of telling mine so I decided to couch it as though it was best for him too. Keep the first chat very short. I tried to be straightforward and the paragraph below is roughly what I said.
"I'm not happy here anymore, and I know you aren't either, so I think it's best if we split up so we can both be happier. It would be nice if we could do that amicably don't you think? Anyway, have a think about it and we can talk about practicalities tomorrow unless you have any questions? No? Okay, I'll go start dinner/wash up/walk the dog/whatever you need to do to get out the room".
Then the next day I asked if he had any questions (nope) so I explained the process approximately, ie you apply online, wait so long, click a link from the court, wait for 20 weeks - but we need to fill in Form E in the meantime and this requires getting pension forecasts and house/car valuations blah blah. Then I ran to the laptop and paid the £600 to get it started.
Keep everything short and to the point for every conversation. Ask if he has any questions about the process. Take ALL emotion out of it and if he starts arguing or anything just walk out the room saying you will discuss it when he's calmed down. Do not respond emotionally.
Good luck. That first conversation can be the hardest because it's more emotional, but afterward keep saying how much it will benefit him (think of ways, ie can watch his sport in peace, won't have wife asking him to do jobs all the time, go on holidays where he wants, won't have to do garden as much if he buys smaller place etc etc etc). I told mine he would probably love a rental as he hates maintenance, decorating and gardening, and how much freedom that would give him - his eyes lit up. We have to live together until the house is sold but I know my husband so I feed him stuff like that, including how he wouldn't have to watch my BBC dramas anymore, how he can watch wall to wall sport, he can get his favourite (stinky) foods all the time, he wouldn't have to keep tidying up, this might be the last time he ever cuts the lawn. Drip, drip, drip but it keeps him onside which makes the process easier to deal with.
Anyway, hopefully that helps.