I understand your predicament Johnny. When I was a 1/3 way through my line training I was asking myself the same question. I continued to ask it all the way through my line training and well in to my time on line. Like in many walks of life, there are 'many ways to skin a cat' so to speak! Descent planning in the 738 is no different. If any pilot is perfectly honest, it all comes with both experience and practice. There are different methods for different scenarios, which you have probably witnessed already. Understandably this plethora of approaches to descent planning and energy management may be leaving you scratching your head. Added to the fact that different LTCs will either demonstrate or 'patter' different methods. They may also interject with something you didn't already know or hadn't already been shown. Added bewilderment when everything is coming thick and fast during your training!
If I am guessing the airline that you are flying for correctly, then I am aware that the preferred mode for descent planning is VNAV. VNAV is a good bit of kit, however it is a mode that functions best when left alone (i.e. without ATC track manipulation). If it isn't managed accordingly, it will give you duff information. 'Sh1t in, sh1t out' so be careful. This is very evident with guys who become fixated on the lateral deviation indicator. Simply flying a different descent speed using LVL CHG in opposition to the descent speed in which VNAV is using to calculate the path will put you out. This along with no or inaccurate information in the forecast page of the FMC (especially when conditions vary considerably from standard atmosphere) and failing to remove level and speed restrictions in the LEGS page (when they aren't applicable) will put you off the path.
The training department in my airline (and possibly yours!?!) focus on VNAV training and ask LTCs to demonstrate its use to cadets. This is because a VNAV PTH descent will allow long periods of the descent to be conducted with idle power (hence saving fuel). Route modifications such as speed changes or vectoring by ATC during descent may see you doing a little inflight adjustment by using LVL CHG or V/S, updating your LNAV track and/or changing your target descent speed in the DES page and then reselecting VNAV. The mode is also preferred because it ensures all level and speed restrictions within the LEGS page are met. Especially important and useful when flying lengthy arrivals into airports like Madrid or Gatwick where ATC expect published speeds and altitudes to be met for their own planning purposes. This is where VNAV is worth it's weight in gold.
VNAV is merely a mode, and like all modes has it's pitfalls. Always back it up by calculating your own track miles from the various sources of information available, whether it be from the FIX page, PROGRESS page or plain old DME. Again, remember the downsides of each. Neither one will give you an accurate figure of miles to run, especially if ATC are positioning you; but it’s always best to have an idea of your own profile rather than fixating all you attention to the lateral deviation indicator! Another piece of advice I would add would be look for cues from air traffic. If you are passing FL180 in the descent and the approach controller clears you immediately to FL60 and starts to tighten up your arrival then he/she is giving you a big hint that they want you down tout suite! If they are taking you away from your preselected arrival and in turn slowing you down then relax a little. Also keep an idea of what other traffic are doing, especially the aircraft immediately ahead. Keep your eye on the wind direction on the ND and consider how it’s direction will affect you as you are positioning for final approach. A thirty knot crosswind whilst downwind can turn into an unhelpful tailwind when on base. Allow for this and manage your profile early, because often ATC won’t. Also consider aircraft weight. If you are heavy you will carry more momentum and require a greater number of track miles for your descent. Plus never attempt to intercept the G/S in clean configuration when over 60 tonnes – you will need the gear early which unfortunately results in all your fuel saving for that sector heading south!
For CDAs, I like to use the altitude capture range arc (often referred to as the ‘green banana’) It is very useful, however it is based on current v/s and groundspeed. So allow for slowing up and configuring. With a bit of practice you will be able to ‘eyeball’ this as previous experience will give you an idea of what is required.
That’s about it. As I say, descent planning is all about practice and experience. Some times it works perfectly, some times it doesn’t. If you find yourself high on profile and above FL100 then use the aircraft’s energy and increase you speed in descent to come back to where you want to be. Don’t always go for speedbrake, especially when 250kts and below. It just makes a lot of noise and vibration to be honest! If it really goes to a ball of sh1t then 220kts at Flap 5 works well or below 210kts at Flap10; both with LVL CHG or VNAV & SPD Intervent. Although you may see from Vol.1 that using flaps as brakes isn’t a widely encouraged method! I say if you need it do it; as will the majority of LTCs.
Above all enjoy and experiment. When on line some Captains will be happy to let you try things out with a little bit of patter or quite simply let you get on with your own thing. Others will happily fly the aircraft for you, regardless of how much experience you may have. C’est la vie!