1. Stalin's seizure of power
Lenin's funeral

2. At the time of Lenin's death there was no clear successor.
A selection of 'famous name' Bolsheviks: Kamenev; Zinoviev; Bukharin.
Stalin
Trotsky

3. Trotsky was not in Moscow when Lenin died
In 1923 Trotsky had come down with a malaria-like disease. He had gone to the health resort of Sukhum and therefore was not at the centre of power during Lenin's illness
Crucially he was not there when Lenin died. This gave Stalin control of the arrangements for the funeral.

4. Stalin's next move
When Trotsky heard of Lenin's death he immediately prepared to set out for Moscow
Stalin told Trotsky that the funeral would be on the 26th Jan and that as there was no chance that he could get back for it, he should continue with his treatement.
In fact the funeral was on the 27th Jan and Trotsky could have made it back for this date.

5. Stalin and Lenin's funeral
With Trotsky absent this allowed Stalin to be centre- stage as chief-mourner.
He walked behind Lenin's coffin and spoke the eulogy. By doing this he promoted the idea that he had been Lenin's closest friend and adviser.
Not only was Trotsky shut out from this he was portrayed as uncaring because he was somewhere from where he could have travelled to get to the funeral in time.