OK... I will quote my "bible" here, FCTM 747-200, for JT9D-7Q engines.
At the end, I will pass along some personal techniques, and suggestions as I am active as check-captain.
xxx
(FCTM) Reverse Thrust
To obtain maximum reverse thrust, after the interlock release, pull the reverse levers up to the stops. Maintain maximum reverse thrust until the airspeed approaches 60 knots. Start modulating the reverse thrust approaching this speed so that the reverse levers are moving down at a rate commensurate with the deceleration rate of the airplane. A reverse thrust level of 60% N1 should be reached by 40 knots. This reverse thrust level may be maintained to a safe taxi speed or a full stop if required. It is important that the reverse thrust levers ve modulated towards the lower thrust settings at the designated airspeed to prevent engine surging.
(...)
Changes of command to the trust reversers while in the mid-stroke between reverse and forward thrust or vice-versa can damage the actuating mechanisms. They require approximately 2 seconds to extend, and 5 seconds to stow to forward thrust.
(...)
The use of reverse thrust below approximately 40 knots should be avoided (unless required for adverse conditions) due to possible surging and foreign object ingestion. - CAUTION - Do not attempt a go-around after reverse thrust has been initiated. Factors dictating this are - 5 seconds are required for a reverser to stow in the forward thrust position, and a possibility exists that a reverser may not stow in the forward thrust position.
(...)
FCTM - Auto Brakes
MAX - This setting should only be used when a minimum stop distance is mandatory. The distance to a full stop will be comparable to the actual certified landing distance on a dry runway. Operation of this setting results in a full 3000 psi being applied to the anti-skid modules. Reverse thrust does not result modulation of auto brake pressure but is additive to the total deceleration level achieved.
MED - This setting should be used for wet or slippery runways or when positive braking is imperative, and for all landings where rollout distance is limited. Operating at this setting limits applied brake pressure to 1800 psi, and will provide a wet or slippery rinway deceleration with optimum anti-skid performance. Thrust reverser operation will result in modulation of the applied brake pressure.
MIN - This setting will provide a moderate deceleration effect suitable for all routine operations. Thrust reverser operation will result in modulation of the applied brake pressure.
(...)
My notes in briefings...
We generally fly approaches at Vref+5 Kts, with 25 flaps for reduction of noise and fuel burn. I stress the use of 30 flaps when runways are wet/slippery for touch down. When landing is assured, at 50 feet over the threshold, it is acceptable to select flaps 30 if approach was performed with 25 flaps. Let the speed bleed off, with no power increase. Further speed reduction from Vref+5 to Vref when 30 feet above runway (start of power reduction and flare) is acceptable, probably achieving a Vref-5 touchdown.
xxx
I personally suggest immediate pull of the 4 reversers upon touch down, which is indicated by deployment of speed brakes. I suggest immediate full operation of inboard reversers (little concern for asymetric power) then careful operation to full reverse of the outboard engines, in case of power asymetry. In case of engine out (such as in training) for an outboard engine, werecommend deployment of opposite engine to idle reverse only. For the case of inboard engine, we recommend partial reverse thrust.
xxx
Minimum speeds for use of reverse, as copied above from the FCTM. My experience has shown that these are "inboard engines" limitations, as the reverse flow of inboards disrupts the inlet of outboard engines, which are located further aft of the inboards. When moduling power to reduce the reverse thrust, I fully abide by the reduction, however I "deduct" 10 knots for the outboard engines... in practice, reducing the inboards as published to achieve 60% N1 by 40 knots for the inboards, but leaving more power ("1 extra knob worth") on the outboards, to achieve 60% N1 by... 30 knots.
xxx
A few years ago, on a slippery short runway in Chicago ORD, one of our 747 ended with nosewheel beyond the end of the runway, because the PF slammed the reversers from high power setting reverser (to a full stop) but got a burst of unexpected foward thrust, after the reversers did stow. So, my personal technique, is to stay with idle reverse, until airplane taxiing speed (and turnoff, if applicable) is achieved.
xxx
Happy contrails