It will depend on the kind of account you want to open, but most will have a type of account that lets you open it with the vouchers and that's all you have to do. Any that ask for a certain monthly amount will likely give a better rate of interest though.
You can only pay a maximum of 1200 in a tax year into these trust fund accounts and with most of them you can do that as an when you wish.
Foogirl is right that up to £1,200 can be put into a Child Trust Fund each year, but the year stretches from your child’s birthday to birthday rather than tax year. Money can be paid in either by a monthly direct debit or by paying in lump sums whenever you want to - maybe after Christmas or their birthday if they’ve been given some money you want to save.
If you’re still a bit confused the Government has a dedicated CTF website you could look at or the money website This Is Money has really useful articles. I'm new to the site so can't post links I'm afraid but if you put ‘Child Trust Fund’ into Google it should bring them all up for you.
Just so you know, I work for The Children’s Mutual - a CTF provider.
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