Not much interest here
You write mirror letters to each other saying you want to sever the tenancy and convert it to tenants in common.
If you have proprty - the land registry are interested and if you goggle sever joint tenancy their site comes up and appendix A give the form you use without a solicitor.
There is a law case somewhere that says almost any document can sever a joint tenancy - except a will - you need notice apparently. so there seems little prospect of you getting it wrong.
[ These joint tenancies are a left over from a by gone age - even in the 1800s their lordships didnt seem to like them. If you watch - who they hell are they ? Laurence Llewellyn Bowen had a distant relation who cleans up from a will, after a law case he doesnt read. Champion v Edwards - two goes 1847 and 1853 - concerns a joint tenancy which hasnt been severed ( so Kitty gets all the cream so to speak )