Editing a template is no different than editing a proposal, except there is no client associated with a template.
For you, there are two ways of creating templates
1. Create from scratch
2. Use system templates
When you edit a system template, it will save your own copy of template and you can customize as you would like to.
NOTE: In terms of controls, layout and experience; it is not very different than editing a proposal, hence please refer to How to Edit Proposal .