Buy a year 1 revision guide, some paper, including graph paper and some colourful felt tips. You can then work through this with your child, thinking of as many colourful and fun ways as you can to explain it.
I agree with irvineoneohone, you do need to follow the modern methods of teaching maths, but any good revision guide, (Letts or CGP for example), will use things like number lines and partitioning, (splitting numbers into hundreds, tens and units), but if you are unsure, most teachers are happy spend a few minutes explaining how they do things. Also look out for any classes run by the school for parents to understand how maths is taught now.
The main thing is to keep it fun, use as many drawings and diagrams as you can to explain and do not expect your dc to understand it straight away. I used to get dd to indicate with her thumb as to what level her understanding was, so she could see, even if she did not yet completely understand, she had made progress.
Most importantly, a child learns by doing, not by being told. So get a workbook with questions, write out questions and also keep stopping in your explanations for your child to figure out the next step. This takes patience, as children will often take longer to think over things than you think before giving an answer.
Encourage them with lots of praise, but make it specific to an example of something they have done.
Pick a time when they want to do maths and stop when they want to stop.
Hopefully you should not get stuck on anything in year 1, but in later years there is always the videos on Khan Academy to turn to. (When the maths seems incomprehensible, Sal has a very calming voice).