From railway station Venezia Santa Lucia, the public service ACTV number 5.1 will take you from Venice Santa Lucia train station to Lido in about 40 minutes. If you want to pass through the Grand Canal you can catch line number 1 instead and the journey will take about 70 minutes.
From railway station Venezia-Mestre, the public service ACTV with line 2 will lead you to Piazzale Roma in Venice.
Once in Piazzale Roma you can reach the Lido with ACTV: line number 6 will take you to the Lido in about 30 minutes, otherwise you can catch the line number 5.1 that takes 40 minutes. If you want to pass through the Grand Canal and admire its magnificent palaces, you can catch line number 1: the journey is about 70 minutes.