I presume your GPS gives elevation. Plot on your map your location and enter the elevation for that point. Continue to may other locations and repeat the process of marking the location with elevation. Depending upon your vertical scale, draw lines that approximate the same elevation.
As an example, let x = 110, y = 120, z = 130
x..................y.............z
...x.............y..............z.......
..........x.........y........z........
.......x..............y.......z.........
.........x........y......z..........
.......x...........y.......z........
..........x...........y...z..........
...............x..y...z............
Connect the xs together, the ys together and the zs together to get the contour.
cp