The Maestral GUI defaults to creating a new folder named 'Dropbox (Maestral)' in your home directory, even if you have an old 'Dropbox' folder from the official client. This avoids conflicts which arise when two clients try to sync the same folder concurrently. If you would like to continue using a folder which has been previously synced by the official client, you will need to rename this folder to 'Dropbox (Maestral)' before setting up Maestral and then choose to merge its content with your Dropbox. Alternatively, you can run the initial setup using the command line interface which allows you to choose any folder name. If you choose to merge an existing folder, Maestral will index and compare its content with Dropbox to avoid unnecessary downloads of files which already exist locally.
Command line
When running Maestral for the first time from the command line, it will guide you through the initial setup to link your Dropbox account, give a location for you local Dropbox folder, and select which top-level folders you would like to exclude from syncing. To sync multiple Dropbox accounts, you can optionally provide a --config-name argument to create a new configuration. Otherwise, a default will be used.
You can add other files or folders manually later, by running maestral excluded add and see which items are currently excluded with maestral excluded list.
GUI
When running Maestral for the first time from the GUI, it will guide you through the setup interactively.