What particular questions do you have?
On a basic level, it's as simple as reading the back of the food label. On a high level, you would subtract grams of fiber and subtract sugar alcohols (although some people claim they affect their blood sugar)
Net Carbs (g)= (Number of portions) x (Total Carbohydrates - Grams of Fiber - Sugar Alcohols)
So if the back of the label read:
Total Carbs: 38g
Fiber: 9g
Sugar Alcohols 6g
and I was eating two servings worth:
(2) x (38g-9g-6g)= 46g of net carbs