There are a few different ways to do this, but one easy way is to count over a period of two hours. If you reach 10 movements, you can stop.
There are many ways to monitor fetal movements, and various protocols exist. One that is commonly used is monitoring fetal movements daily starting at 28 weeks of pregnancy. Each day, a pregnant woman should aim to feel 10 movements in two hours. If she reaches 10 movements before two hours, she can stop counting. However, if two hours have passed without 10 movements, she needs to alert her doctor or midwife for further evaluation.
Many women will do this first thing in the morning, while others will do it a time when they know their baby is most active (interestingly, between approximately 10pm and 2am is the most active time for many!).