I just finished roofing both of my shed buildings. The process went reasonably quick and I'm not a fast worker at these types of things. I tend to putz along.
Roofing Instructions
Every single one of the roofing bundles I got (30 year comp shingles) had detailed instructions on the packages. It was very helpful as I'd never roofed anything before. There was info on how to start the shingles (cut off the three tabs (I set them aside for in fill elsewhere) and 5 5/8" from one end of the first starter course). There was also detailed info on how to do the patterns and layout. There was also info on laying up the roofing felt and what was exactly recommended by the manufacture.
So, take a look at the bundles of roofing you're going to buy and see if they have instructions on them. If not, check the manufacturer's website for instructions. You need to built it according to the instructions if you want to be covered by the warranty.
If you don't want to be covered by the warranty then you can build it according to the photo above.
Flashing
As noted, you'll need flashing along the eave. But also a drip edge at the rake, factor that in, too. On the rake, over lap the flashing at least 2". For the eave and other horizontal lap joints, over lap min 6". FWIW, my lumber yard had what they called starter flashing for roofs. It was a stick of flashing about 4" wide with a 1" bevel or angled facet to it on one side that had a hemmed edge. That would go down first. Then the starter course of the 3-tab shingles. Then you'd start putting down the normal roof singles.
Got so excited to start roofing I almost forgot
to put on my starter flashing, had to peel back
the first course of felt do put it on. The spring
clamps held it in place until I could nail it.
Ice and Water Shield
If you REALLY are not going to heat the building then you won't need the ice and water shield, especially if you have a decent slope to your roof. Ice dams are formed when roof is warmed by a heated structure. It melts the snow directly above the heated rooms. The water runs down the roof (as it's supposed to) but then freezes when it reaches the eaves that are hanging out over air and are not heated. Over repetative heating / freezing cycles this can build up thickened layers of ice or ice dams out on the eaves and back water up under the shingles.
Where as if the building is definitely not going to be heated then there is nothing that's going to cause the snow melt over JUST one portion of the roof.
Yes, you might get snow melt on an unheated roof during the day, but it'll likely be consistent across the entire roof with a consistent roof temp. Once things freeze up again at night it'll freeze evenly over the whole roof so you won't get the build up of ice damns in any one location.
30lb. vs 15 lb. Felt
Up do you. I went with 15 lb. felt but double layered it by over lapping each course 19". This was based on the shingle manufacturer's instructions on the roofing buldles. And this was for a "low slope" roof condition that I have on my sheds (below 4:12). Above 4:12 roof pitches they said to just use the 15 lb. felt and over lap it 2".
Snapping Chalk Lines
I didn't do this. . . and I didn't have any problems with alignment with my coursing.
I thought I'd have to do the chalk line thing, though. Turns out that when I started unrolling the 15 lb. roofing felt it already came with white horizontal lines on it: 2" in from each edge (easy to align the 2" over lap), a pair of lines in the middle 2" apart (easy to do the 19" overlap), and I think one line between each of these pairs. The result was built-in horizontal rule lines every 6-8" or so. It makes it very easy to eye-ball each course of roofing shingles. Sometimes they fall directly on a line. Other times they're close enough you could tell if you're off or not.
Granted, this presumes that as you roll out each course of roofing felt that you keep it pretty even and not let it get wonky on you.
To give you an idea of how well this all worked out. . . after installing all the coursing all the way up a 24 ft slope of roof, once I reached the top it turns out the top course was pretty much dead on. Might have been off by about 1/4" but that's not going to be noticeable from the ground at all.
Note the horizontal lines on the 15 lb. felt
Also note the far right edge has the drip
edge flashing fastened down.
The courses looked even to me and I
didn't bother with the chalk lines.
Nail Gun vs. Hand Nailing
Not wanting to buy a specialty nail gun JUST for this I opted to hand nail. Not as fast but it wasn't really that slow once you get into the rhythm of the roofing install. No doubt at all a nail gun would have been awesome here, but it wasn't really a worth while investment in my view.
FWIW, my rank amateur self could have finished roofing the 12ft x 24 ft roof in a single day. AS it was, I was working on it for about 1-2 hrs a day after work throughout the week.
Work Station
Up on the roof I had my "work station" set up. I used a scrap piece of plywood about 6" x 48" as my cutting board. A 24" carpenter's square worked great as a measuring tool and cutting edge to get square cuts on the shingles. Then there was the tub of nails, hammer, and utility knife. That was all I ended up needing.
Save Your Scraps
As you cut pieces off each 3-tab piece to do your pattern lay-up, hang on to the scraps, they can be used on the other side of the course to fill in to the edge of the roof. Doing this I ended up with VERY little waste.
