As Chip says

Its much easier with the engine out. As you can rotate it on the stand and do the big end (conrod) bolts easier. Like everything on a car its just bolting stuff together. And its not a particularly difficult task. Mainly time consuming.

Though I would say dont attempt it if you arent confident. Have you carried out a wet and dry compression check and a cylinder leakage test yet? It might not be the rings remember.