The best way that I have done is with a piece of string. Run the string along side their body, then cut it at the end. Same for around the snake at the largest part, just wrap it around the snake and then cut. If you want to weigh them you can buy a kitchen scales that weighs in grams at any Walmart or Target store for around ten dollars.
i already know how much my girl sam weighs over 5 pounds and i got a scale that weighs in grams ounces and pounds from local head shop