This pro takes care of gaps by applying baseboard trim, a long thin piece of horizontal wood with an ornamental profile cut into its top edge.

How to remove a baseboard with a trim puller You don’t need any special tools to carefully remove baseboards, although they do help.

Here's the whole step-by-step for how to install baseboards and crown molding—note that most of the work happens in the planning phase. The actual install takes no time!