Jellycats4life has good advice.
When we were preparing DD for this, we didn't directly start solving these problems (well, we did first, but didn't work!)
We started solving easier problems eventually leading to more complex word problems typically seen in exams
We looked at statements and expressions that can converted to a mathematical expression.
For example, we taught first that expressions like 'more than', 'increased by', 'on top of' etc lead to addition. (So what is 8 more than n? --> 8 + n)
While expression like 'reduced by a factor of', 'split between', 'halved' etc can be expressed as a division
And so on for multiplication and subtraction.
DD then solved simple problems like -
What is 7 more than n?
'b' split between 8?
'c' decreased by 7 ?
etc.
We then started solving simpler word problems with such expressions (but still single step problems)
And once DD was comfortable with these, we moved to more complex multiple step problems etc. (see Jellycats4life's reply for how to apporach these)
If you are doing DIY (and you are not a professional teacher), I would suggest using some book that take such an approach and has plenty of exercises. We were using AE tuition's 11+ maths workbook. But there are plenty of others that take similar incremental approach.
Good luck!