Two simple general techniques beneath to kick off, for aircraft with manual backup in the event of a single governor/FCU failure. To my mind, the appropriate choice depends on what previous experience you have, how clear the cockpit indications are to easily differentiate between #1 and #2 engines, and whether the RFM specifies any differential TQ limits.
Note - before attempting either, make small collective inputs to ensure you have correctly identified which engine responds in TQ to collective inputs (automatically governed), and which does not (fixed/un-governed/manual).
1) Use manual control on the un-goverened engine to “match TQs” throughout (assuming TQ limited). This minimises the TQ differential between engines and if managed well reduces the likelihood of the automatically governed engine from approaching it’s upper or lower limits. However, the downsides are regular throttle inputs are required which can reduce capacity to fly a decent approach, which in turn may lead to larger collective/throttle inputs being required, exacerbating the problem. Also with both engine indications constantly moving there is the potential for inputs in response to the incorrect engine if the cockpit indications are not entirely clear which engine is in manual - these can be very confusing and rapidly lead to engines approaching limits.
2) set a nominal power on the manual engine appropriate to the phase of flight (low power for descent, mid power for cruise, higher power for climb), and allow the automatic engine to vary within it’s allowable limits for small fluctuations. Before lowering the collective to descend always roll off throttle a little, and never let the automatic engine reach <10% TQ, if it reaches 0% and you continue to lower the lever it cannot reduce TQ any less, and the reduction in drag from pitch reduction will lead to an engine/Nr over speed (or perhaps activation of the over speed protection circuit if fitted).
To a great degree the manner in which the approach is flown is more important to the outcome than excellent throttle control. My advice is fly a slightly shallow and slow approach. One really common error is over concentrating on the throttle control and being fast or steep at the end of the approach. Either case leads to larger than normal collective inputs which in turn requires corresponding throttle inputs. If you fly a slow and slightly shallow, constant angle approach, and get on the low speed side of the ‘power required for level flight’ curve early (generally <40-50kts by approximately 300ft) then throughout the remainder of the approach you progressively lose translational lift, meaning incremental small increases in collective and throttle until close to hover power is achieved which makes life very easy. Personally I try to keep the manual engine TQ just beneath that of the automatic engine when approaching the hover but that’s just preference for uniformity in indications. The single most common way to make life hard is to end up fast, with a flare at the bottom (large lowering of the collective), followed immediately by a rapid loss of translational lift leading to a large raising of the collective - avoid doing this...