Hi,
Basically, you find out how many grams of carbohydrate there are in the food in question (the packaging is often very helpful),, then you add all the foods in your meal together, and get a total of grams for that meal.
IF there is a label on the food, then that is the easiest way. Just see how many g are in 100 g of the food, and guesstimate the size of your portion. So if 100 g has 40g carbs, and you are eating 50g then that is 20g of carbs that you are eating.
Often the packaging has the carb content listed by portion as well, so a slice of bread may be 22 g, or 16 g, depending on the brand.
Alternatively, if your food doesn't have a label then you can go online to somewhere like Tescos supermarket's website and look the food up there. They almost certainly show the nutritional values.
Another way is the buy the book (or App) Carbs and Cals. It shows a picture of each food, in different portion sizes, and tells you the grams in each size.