The description of the walk should specify the location of the walk start point. It should also contain the grid reference for the start point and may contain a post code. If you look at the walk description on this web site, you should see a Google Maps link to the walk start point as well (but make sure it is the start point and not the meeting point you are looking at!).
If you have a SatNav and a post code is provided with the walk description, then typing the post code into the SatNav may be the easiest option for finding the start of the work. However, it is important to realise that post codes represent an area, rather than a point, and become less accurate the further away you get from built-up areas. For walks in mountainous areas, using a post code and a SatNav may take you a long way away from the start point and you may end up missing the walk. If the walk description does not contain a post code, the walk leader has deliberately removed it from the walk description - this is a strong hint that the post code is unlikely to get you to the walk start point, so you should probably not try to determine the nearest post code using other means. However, do not assume the presence of a post code is a guarantee it will work - they are added automatically by the Ramblers Walks system.
If a SatNav and post code are not an option, you or one of your passengers will have to navigate. Having a passenger do the navigation should make life a lot easier. In this case you should work out your route in advance (this is a good idea even with a SatNav).
If you are driving to the meet point, make sure you know how to get to the walk start point, as you may be the one driving and the walk leader may not be at the meet point.
If you have questions about how to get to a walk start point, contact the walk leader in advance.