Marx amended the manifesto shortly after the Paris Commune and said that seizing the state is not a good idea, and that instead one should try to build a society with as small government as possible using principles of direct democracy.
He said this because for him the Paris Commune was proof that initiating socialism is possible without State power. Since he profoundly disliked the State, he thought that it was better to abolish the state and instead implement a government of direct democracy and freedom.
Lenin, although he had read this and agreed with this anti-statist assessment, judged that it was not possible to implement such a strategy in Russia, because Russia was too big to rule coordinately in such a way, not in size of population, but in physical size and disparity. This was not necessarily an issue, but given the war that was raging at the time he decided that it was too risky to fragment the country at that point. He was also expecting that the rest of the world would see socialist revolutions, which was not unexpected at the time, but this was very optimistic. Since that did not happen, the state of the Russian Empire was maintained. This was, obviously, a huge mistake. He realized as much later, but did not see a way to fix that issue immediately. As he was making plans to prevent Stalin from taking power however, he unexpectedly died of a stroke at a very young age. All in all, Lenin, in the founding of the USSR made a series of mistakes that were so obvious that he eventually realized them. If he would have attempted to rectify them is an open and unanswerable question.
The conclusion from all of this, that most socialists realize nowadays, is that attempting to install an autocratic Leninist state is a huge mistake. This is why most modern socialist experiments have been centered around libertarian socialism, that attempts to implement the ideas that Marx was persuaded by after the Paris Commune.
In conclusion, you should read Marx's preface to the communist manifesto, in which he makes the points I have made above and warns against following the manifesto, claiming that history has proven him wrong and that it should not be considered as anything more than a vestigial document of nothing but historical interest.
He also warns that his ideas on how to implement a socialist society are, unlike his analysis of capitalism, nothing more than suggestions and that one should let them be empirically proven or disproven.