The fastest way to sound confident is to slow down slightly and speak in shorter, clearer sentences. This page shows exactly what to do, and what to avoid.
Not slow, controlled. The biggest issue is usually speeding up mid-answer when you’re thinking.
1) “My approach is…”
2) “First…, second…, third…”
3) “The result is…”
This is the easiest way to reduce rambling and “thinking out loud”.