If this is your first time tucking, it might feel uncomfortable, but should not feel painful. This is a comprehensive guide, providing technique and what to expect:

https://fenwayhealth.org/wp-content/uploads/Trans-Health-Tucking-Guide.pdf

Put your legs through the legholes and stand up, pulling the waistband right below your hips. Gently tuck your penis back between your legs while inserting your penis into the pocket feature (if applicable). Pull/tug the panty upwards (not too rough!), from the center front and the center back of your body. Smooth out the waistline and legholes.

For an even flatter look, you have the option to tuck in your testicles into your inguinal canals. The inguinal canal is located in your lower abdomen. The opening should feel like ring with a slight indentation. Place each testicle in the corresponding side. This should not be painful. If you feel pain, stop and try again later.

Always use caution when sitting down while wearing a tucking panty. If you experience discomfort or an erection, loosen or temporarily remove the panty.

Pressure causes diamonds— however in this case, constant pressure to the penis and testicles may cause pain and/or infertility. To receive proper medical advice, please consult with your doctor before using the tucking function.