Have a look at
http://en.wikipedia.org/wiki/Marking_out http://www.clag.org.uk/workshoppart02.html
Google throws up many other options.
At the risk of stating the obvious:
There are 5 stages to making anything:
1. Mark out the parts
2. Cut out close to the marked line (leaving a margin for accuracy.)
3. finish to size (using finer tools, files, emery cloth, etc finish exact to the lines.)
4. test the part (technically, in small scale manufacture, this is most likely to be a quality control check of dimensions with suitable measuring instruments or a fit check against another part)
5. Assemble the parts.
If you have the ability to use CAD/CAM equipment then drawing the part on the CAD system equates to marking out and the cut/finish to size is done by the CNC machine. - Even then the machine will rough cut and then finish to size. As the machine actions are more or less totally repeatable (!) this is a form of Quality Assurance i.e. a system to make sure you don't make expensive mistakes (but we don't do that at home do we!)
Th is extracted from things I tell the kids at school! so sorry if rather basic.
As an aside I recently discovered the bottle of engineers blue I inherited from my dad has finally come to the end but I am unable on a browse of various ME supplier sites able to find a stockiest where do yu lot in the UK buy your blue.
In the mean time I guess a felt tip marker will do.