The bottom-up approach looks at stock triggers straight away. The company is the starting point. The bottom-up approach is based on the premise that good companies can successfully create wealth even in tepid markets and when the economy is not performing great. This is true if you look at specific instances like TTK Prestige, Eicher and Indo Count which actually gave phenomenal returns at a time when the markets did not show any great outperformance overall. The difference between the top-down and bottom-up approach is that the latter believes that stocks can be economy and sector agnostic.
What is normally the starting point for the bottom-up approach? You have triggers like company news, mergers, acquisitions, big orders, shifts in industry structures, sharp rise or fall in profits, increase in market share etc. All these can be triggers where the bottom-up approach gets triggered. This is followed by the mere ratification by looking at industry-specific and macroeconomic factors.