Hi,
Normally for inflight diversions they're looking to see you make a quick guestimate of things so you can start the diversion, then you can get more accurate once you are under way.
Typically, this means you need a bearing to fly before you can set course. I was always taught to eyeball this on the chart. Make sure you know where you are currently, figure out where you want to fly to and estimate the True Heading. Then add on your mag variance. You can also get the Magnetic heading from a handy VOR rose that may be on the chart by visualising the course on the rose and reading off the bearing. A pencil or pen sometime helps do this. Then you can add a drift due to wind to this. Again, unless you have a GPS system that tells you the real wind at your height then this will be another guess (it's effectively a guess at the planning stage anyways).
The examiner will be looking to see that you can make a good 'guess' at the heading to fly. After you've woked out the heading you can work out an estimate for GS and ETE (leg time). These can be done once you have taken up the bearing.
If you notice yourself offtrack, then make suitable adjustments as you would during normal navigation.
It will help if you do a few practice diversions in the comfort of your living room. Try and 'guess' everything, and then try it in full Nav planning mode. After a few attempts you'll find your guess is very close to the real calculated plan.
Have fun, and good luck in the skills test.
Cheers,
DB.