I just put plastic over her mattress and a couple of thick towels over that and then her fitted sheet as usual and had spare bedding and pyjamas at the ready.
I think if a child is trained in the day, it's usually a good indicator that there's no medical/developmental reason that night time training can't commence, that said though, some kids just don't wake when they need to.
Marigold has some great advice, having a nappy on is a bit of a psychological barrier in that they have that little bit of a 'security blanket', so if you can ditch the nappy it's a good start. Another thing I found is once I stopped my DD from going to bed with a drink she would wake with a substantially less wee laden nappy, and that really helped too. It's a bit of work, but a good mattress protector, some spare sheets/towels etc at the ready and being prepared to have some wet sheet changes for a bit and hopefully with all that the penny drops and then dry nights.
Make sure you have a light source on in the toilet and no obstacles once they're in the toilet.
I don't think you can train them at night to be honest, I think it's something that just happens naturally, until they are waking up dry every morning id leave them in pull ups. We were lucky with DS we took him out of night nappies at 3.5 after he'd been waking up dry continuously for 2 months, he's not had an accident yet, he's now 5.5.
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