The simplest navigational technique is piloting, we all do it unconsciously whenever we go for a walk: it is the art of determining your location by visual sighting of landmarks. On a boat, piloting can be quite precise with just a few simple tools: a chart, a bearing compass (a compass which can be aimed precisely like a rifle and read with high accuracy), and some simple drafting instruments.

The fundamental piloting operation is shooting a bearing, which provides you with a Line of Position (LOP). In practice, this involves using your compass to determine the direction (bearing) to some visible landmark that is also plotted on the chart such as a lighthouse or smokestack. Move your parallel rulers to the compass rose printed on the chart and align it with the bearing you've just measured. Walk that bearing with your parallels, maintaining their orientation to the map grid, until the edge of the ruler touches the symbol on the map location of that landmark. Draw a line on the chart along the ruler, and that is your LOP. You are somewhere on that line. Repeat the process with another landmark, and where the two LOPs cross is your position, or fix. If you have a third LOP, the size of the little triangle where they intersect gives you an an idea of the precision of your fix. In areas where there are few landmarks available, the authorities often place charted aids to navigation such as buoys or range markers to make up for it.

There is more to it, of course, such as bow and beam bearings, tangent bearings, sailing range markers, running fixes, and other techniques. But that's basically all there is to it. For example, if you have radar aboard, you can use the distances to objects on the chart instead of their directions. Radar gives very inaccurate bearings, but very good ranges. In this case your LOPs will be arcs of circles centered on the landmarks, of radius equal to the range as given by the radar. LOPs can also be determined with radio direction finding (RDF) equipment, or by electronic systems which employ similar principles, like LORAN or GPS.

GPS makes all these skills obsolete, but if the equipment should ever fail, for whatever reason, and the mariner doesn't have any alternative skills, he will be effectively blind and helpless. Piloting techniques are very simple, but they also require judgement, which can only be acquired through experience. When unqualified boaters rely too much on their GPS, accidents occur even when the gear functions perfectly.
The joke is "The main purpose of GPS is to tell you EXACTLY where you've run aground."