How do pilots adjust trim on the B777, where the control forces are simulated?
The basic idea, as the FCOM says, is to simulate control column forces (as in 'conventional' aircraft) so that the pilot knows that he/she is out of trim. So, once the aircraft has been trimmed, there is no force on the control column- the pilot stops trimming at that point.

Boeing 777 uses a feedback actuator to give an artificial feel to the pilot emulating conventional aircraft- this is no accident, as Boeing B-777: Fly-By-Wire Flight Controls notes:


  The philosophy employed during the design of the 777 Primary Flight Control System  maintains a system operation that is consistent with a pilot’s past training and experience. What is  meant by this is that however different the actual system architecture is from previous Boeing airplanes, the presentation to the pilot is that of a conventionally controlled mechanical system. 


The control columns (yokes) have feel units, which provides tactile feedback to the pilots just like conventional aircraft:


  A gradient control actuator is attached to the two control column feel units. These units provide the tactile feel of the control column ... (This) is consistent with a pilot’s experience in conventional commercial jet transports.


The image below shows the Boeing 777 yoke with the trim switches. 



... The buttons on the right side of the yoke control the trim surfaces for the elevator.; image from wired.com

This trimming system works just like the conventional aircraft- as you noted "they hold the required attitude using the yoke, then trim the aircraft until no force is required to hold the yoke in that position". Note that, though what happens internally is different from the conventional aircraft, what is presented to the pilot is same. From An Analysis of the Boeing 777 Fly-By-Wire System:


  ... the pitch trim system ... is used in the exact same way, that is, should the pilot find himself pushing the control column forward to keep his desired flight condition, pushing the trim "up" ... will gradually reduce the required force on the control. The opposite goes if the pilot is pulling back on the control. Just like in any aircraft, the ship will be "in trim" once the required force on the control column is zero.