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The USGS maps cover 7.5 minutes of latitude and 7.5 minutes of longitude. The four borders of the map each have index marks at 2.5 minute intervals. If you connect the index marks with pencil lines, your USGS map is now divided into nine quadrangles.
Each minute of latitude or longitude is further sub-divided into 60 seconds: so your 2.5 minute grids can be sub-divided into 60+60+30=150 second increments (but don't try to draw these). The lines of latitude are parallel and equidistant, and each minute of latitude is equal to one nautical mile (useful to know if you're navigating the ocean). But now it gets interesting… the quadrangles are slightly wider at their bottoms than at their tops (because lines of longitude converge at the poles); so to plot your longitudinal coordinate, no fixed distance template will work.
What you need is a ruler that is divided into (in the case of your 2.5 minute quadrangle) into 150 equal increments, with a length that is slightly greater than the distance between the index lines. The three-sided engineering rulers (with six scales) will usually have at least one scale that will work for you.
Here is an example of the procedure (Graphic Version):
If the point you want to plot is at 105 degrees 31 minutes 47 seconds West; 38 degrees 48 minutes 33 seconds North - we'll start with finding the longitude.
The point will be between the two longitude index lines 105 32' 30" W and 105 30' 00" W. Knowing that 2.5 minutes comprises 150 seconds, 105 31' 47" will be 60+47=104 seconds to the west of your 105 30' 00" line. Taking your graduated straight edge, place the "0" on the eastern index line and angle the straight edge such that the "150" is on the western index line. Make a pencil mark at "104" on the straight edge.
Now slide the ruler above or below the point you've made, check your alignment at the index line, and make another mark. Now draw a light north-south reference line connecting the two marks you've made.
Now repeat the process for the latitude portion of the coordinate. If the lower (southern) index line is at 38 47' 30", align the "0" there and the "150" with the 38 50' 00" line, and make a mark at "63". Using the upper and lower index lines, sliding your ruler side to side to make your two marks and drawing an east-west reference line.
The point where the two reference lines intersect is your coordinate. Hint: to ease the math burden, plot and create your own index lines at one-minute intervals.