You don't need a ruler to sketch straight lines and actually the point of sketching is to NOT use a ruler. In this video I demonstrate techniques that enable even the most fumbled fingers of individuals to sketch nice straight lines:
- Do not look at the pencil tip
- Quick lines, rather than slow methodical lines
- Create an end target
- For longer lines, create intermediary targets
- Feathering can help, but is not recommended for technical sketches
- Rotate paper when needed
- Practice, practice, practice