Dirt simple
All I do is scribble next to the box in the instruction manual the date that I completed a step. Now that I have the website, I don't even do that.
I also have a file with all my reciepts in it. Buy a part, throw the reciept in the file. I've never looked at it, and I have no idea how much I've spent so far.
So I don't know exactly how long it took, or precisely which days I worked on it, or exactly what I spent but who cares? It'll fly the same if it takes me 1000 hours or 5000 hours.
It's just an airplane
My digital camera (I would imaging most are like this nowadays) embeds date/time information in the EXIF header in the picture. As long as you have the date set right (which I didn't for the first month), keeping a log is TRIVIAL. Every now and then when you're working, just snap a picture of some interesting thing. Make sure you have a few pictures of yourself in there too every now and then.
At the end of the day (or when your camera fills up, or when you feel like it), just dump it all to a folder somewhere and you're done. Each photo has the date and time embedded in the header information. Sounds like a log to me! You can come back later and split them up by day. There are neat tools that will examine the EXIF information and do stuff like that automatically.
(I should mention that if you go this route, you should really have a place where you can scribble down some notes by hand for days where something interesting happened or you deviated from the plans/instructions, but the pictures take care of all the mundane heavy lifting...i.e. put the skin on, trimmed the stiffeners etc..).
Alternately, you can do what I do and use the EXIF information to auto-generate web pages for you. I just dump all the files in a folder, run a PHP script on it and presto, instant web updates. I go back later and fill in comments when I'm able. I'd be happy to send you the scripts I wrote. If you wait a week, it'll auto-resize all of the pictures as well, making it even simpler.