There is a link to blood glucose testing on the home page.
But in simple terms, you take a reading before you eat, then test two hours after first bite. If your two hour reading is over 2mmols higher than your pre meal reading then, some food or combination of foods is not to be eaten.
Always keep a food diary, as this will show how well you are doing and you will see trends in months to come, always, list everything in your diary, as this will also help your dsn, GP, to see how you are doing.
If you haven't already, have a look at the low carb forum, there are some great ideas and recipes for you.