Usually sitting at the begining then standing at crowning stage but still its according to the operator 

As per Accoucheur's discretion.

When apply the forceps , accoucheur should sit down , check position of the blades , then test traction and pull until crowning while sitting. Episiotomy should be done while standing and crowning appear.

always standing like operator do

Some books mention sitting position and some does not mention anything.

In a research published in Obstet Gynecol (2005), forces generated from the sitting position in particular can often exceed the preferred range (30-45lb). The mean maximum traction produced by men in the standing and sitting positions was 69.5 and 85.8 pounds, respectively. For women, the mean maximum force generated was 45.5 pounds in the standing position and 61.3 pounds in the sitting position.

I never conducted obstetric forceps in sitting position, as I was trained like that.

Those who are trained in sitting position, they have adapted themselves to generate adequate force. So there is no need to change their current practice of sitting position.

 Standing like operator.

Always standing, because first you must pull down to bring the subocciput under symphisis  (sometime you must use a higher force) and after that you must pull up the head (to respect the natural mechanism of delivery). If you are in sittig position you don't have facility in movements. I think that maximum traction produced is higher standing.

I have no idea how does the midwife can help you in a sitting position:)

I'll be standing up - more alert that way. I have not  come across any hard evidence for and against

Stand steadily on my feet!  If I need to use medium+ force I can do so using my arm in a fysiological position and I can easily change the direction of the forceps shafts upwards and follow the pelvic curvature, at the same time support the perineum. I can also have constant eye contact with the mother to guide her pushing so that she can be delivered smoothly and be included in the birth of her baby. Maybe a tall colleague would chose otherwise, no need to be absolute about this.

 Sitting has always worked best in my experience. It allows a solid base, and more natural downward traction, using a well-controlled downward pulling in the vertical axis, rather than pushing, to bring the head under the symphysis pubis. Also, one is closer to the plane of the perineum for visualization of its distention and potential need for episiotomy. I often stand as the the baby is crowning, to accomplish removal of the forceps blades and the for the remainder of the delivery. I agree that everyone should develop their own method, whether sitting or standing, and be flexible. In more austere settings I have occasionally needed to kneel on the floor.

I stand allowing full movement. works for me especially in rotational deliveries.

I prefer sitting position as it gives good control in giving downward traction and the force required to deliver in a correct manner.

I prefer sitting and teach sitting. it limits the amount of traction better, particularly for male accouchers. the height of the bed is important though to allow correct traction axis and many (like Prof Walker above) prefer standing to afford better range of movements.

I practice sitting and encourage juniors to do so. It prevents undue traction being applied, gives better control and appears as a less traumatic procedure for the patient and the companion. 

I prefer sitting position. Gives better control in force applied to downward traction, allow correct traction axis, with less trauma. I  stand as the the baby is crowning, to accomplish removal of the forceps blades and the for the remainder of the delivery.

Sitting on a stool ( with no wheels) is safer for the practitioner as the risk of falling backwards is minimal. It is also safer for the baby as you have your lap as an extra layer of protection should the delivery become rapid. In terms of angle and force , then either sitting or standing should not make a significant difference but junior doctors are encouraged to be seated. Personally I prefer standing and raising the bed up .

The practitioner should be sitting on the floor and the woman should be squatting in a full squat with feet flat on the floor, with each  hand pushing on a low night table- one on either side of her, and she will certainly push the baby out, without forceps. 

I think whatever position you feel most comfortable. Personally I usually stand but as long as you have a solid base which does not move I honestly don't think it matters. I stand so that the patient sees me and I can talk to them whilst performing the delivery but we also use a wedge so the patient is propped up a little and not lying flat.