Pending what machine you have access to, you will either be using a chest-supported machine or a standard foot-plate variety. We will talk about the foot-plate machine here.
Assume a strong lifting position: core and glutes activated, shoulders braced, chest up, knees slightly flexed, and eyes fixed ahead.
Hinge at the hips while maintaining a neutral spine.
Maintain a slight knee-bend throughout the movement.
All T-Bar machines are different and will employ various handle variations.
Pronated Grip
Activate your scapulas and upper back to raise the the bar towards your chest.
Imagine you are pinching a quarter between your shoulder blades.
Do not actively pull with your arms, they remain passive.
At the top, you should feel like a scarecrow.
Neutral Grip
Use your lats and mid-back to pull the bar up & around your body.
Don't let your upper traps shrug.
You will be in a more vertical position.
Your arms are a lever, they remain passive in the exercise.
Lower the weight along the same path.
*Tips*
Your torso remains tight and in the same position from start to finish. Don't rock your torso up/back to move the weight.
This takes tension off the proper musculature you want activated in the movement.