There are 3 vectors in the wind calculation:
- Wind speed and direction
- Heading & IAS (or more accurately TAS)
- Track and ground speed
If you know 2 of these you can calculate the third, but remember that a vector has both magnitude and direction.
In the usual flight planning problem, the wind velocity vector is known (from forecast), but the other two are actually unknown, insofar as you will know your desired track but not the ground speed and you know your IAS but not your heading.
This is why the problem requires an iterative 2-step (or more) approach to solution - the "jiggle".
An approximation is made, based on the assumption that heading and track are initially the same. It is then necessary to enter the revised heading to refine / confirm the calculation. In strong winds or large angles between track and wind direction it may be necessary to do this more than once.
In effect you are solving the problem: given wind velocity, heading and IAS, what is your track and ground speed? You are assuming a value for heading, and then checking that the assumption is correct by solving the problem and ensuring that the answer is your desired track, and iterating as required until it does.
Also remember that you are always blown from heading to track, so if you are ever unsure about whether to add or subtract the drift, just look at the direction of the wind on the map and then see how it relates to your track line - which way will it blow your aeroplane, and will you need to increase or decrease your heading to compensate?!
Stick with it, it gets easier!