It can be work, but it's sorta fun to me.
1. download quadrangles
2. draw in gridlines using graphics app (graphicconverter for mac)
3. figure out the four corners (utm coordinates) of map (most will include more than one quadrangle)
4. crop each quadrangle to just what is needed
5. open new image, paste all cropped images into it, line them up, crop out excess, save
6. figure out how many sheets of paper the new map will need to be printed on, then crop again for each sheet
7. type in utm coordinates and verify they are correct (see original quadrangles)
8. print each sheet
I don't worry about scale too much. If you are solo and/or offtrail hiking, you really need 1:24k. The typical scale on my sheets is around 1:50k. You can calculate your map scale by measuring the distance between the kilometer marks and plugging it into the scale calculator at Oregon State University.
It would be nice if Hillmap (or somebody else) built this functionality into their website so you could just type in the UTM coordinates of the four corners of your map and download the map with a UTM grid already overlayed. I don't think it would be that hard for a programmer to do this. GMAP4 has added a UTM grid overlay feature.