There are basically two ways to create new networks: First, based on the references of a given paper. This could be a review or just an interesting paper you'd like to dig deeper into. Either click the + button at the top right and enter the digital object identifier (DOI) of the source article, or click the blue reference-button of an interesting citation in the Refs-column of the tables if you already have opened a network.
The second way to create a new network is to scan a text file for DOIs, which allows you to create your own unique lists of input articles. For example, this could be a .ris / .bib / .json file exported by your reference manager (e.g. Zotero or EndNote). In theory, you can scan any plain text file (PDFs and Microsoft Office files don't work). However, if the DOIs are embedded in sentences or URLs and have a trailing fullstop (.) or slash (/), the system will have difficulties extracting them correctly and might not find all of them.
Up to 5 tabs can be open at the same time, representing a different local citation network each. Only one query can be processed at a time, which is why some buttons are disabled when a query is still going on. Microsoft Academic seems to be able to process quite large numbers of references (I've successfully tried up to 500) but Crossref's API returns an error when trying to process too many references (depending on the length of the search string, around >110 DOIs).