Gas furnace (gravity system, no motors, no fans) - pilot light is on 24/7.

Would like to install electronic ignition.

Need installation instructions and schematics.

You need to chose a product, which then you will find the installation instructions, etc.

Here is one made by Honeywell:

http://customer.honeywell.com/techlit/pdf/63-0000s/63-9415.pdf

Check with local codes, many areas require a license to work on natural gas systems, and electrical wiring.