Just use your thumb and middle finger to hold the bigger tube and put your index finger on the smaller tube (applicator).

 Or, take the applicator with your thumb and your third finger, and push the tube inside with your second finger.

Sorry for my english.

I'd go with an applicator free one like OB - I've done those one handed. Just put your middle finger in the dent of the tampon, use your index and ring fingers to spread the labia, and insert!