The most effective way to combat congestion with Eucalyptus Essential Oil is steam inhalation, which allows the oil to react with mucous membranes, reducing mucus and loosening up congestion. It’s important to note that eucalyptus oil is strong, so you only need one or two drops to achieve the desired effect. 
To create steam inhalation: Place one to two drops in a small bowl of steaming hot water. Drape a towel over your head to focus the steam and stand with your head at least an arm’s width away from the bowl. Close your eyes and breathe in deeply, allowing the vapor to enter your nose. Inhale the steam for five to 10 minutes.