You have three options.
You can heat the milk in a heavy bottom pan until it caramelizes. Takes forever and usually burns!
You can put the unopened tin in a pan of boiling water and simmer for about 2 hours. It can explode,It did happen to me once and the mess is unbelievable.
Or do what I do and buy the Nestle caramel in a tin.
Good luck.
You might like this one as well.......Key lime pie
Crushed ginger biscuits and melted butter for base.
Zest and juice of 4 limes
1 can condensed milk
10 fluid oz double cream
mix milk,cream and lime in bowl ( beat it if you want but i generally just stir well )
As it thickens pour into base and chill.
I make it day before as the flavour seems to improve on keeping.
It's seriously yummy