If you struggled with the Manifesto then Principles of Communism will be a lot easier. It actually is a draft of the Manifesto written using the catechism convention, so it’s one question, one answer, and it goes really in a simple way about what are the social classes, the relationship that distinguishes for example the serf and the feudal lord from the proletarian and the bourgeois and so on. It’s actual a really good idea to read it before the Manifesto and a lot of editions include it as a preface. Don’t worry about not understanding everything, though, the Manifesto is a work that simple and profound, short but dense, and that you should read multiple times throughout your life to get the most of it, it condenses some really insightful thoughts in a way that’s accessible but that will also reveal more of what it has to offer the more you read Marxist literature.