Export your noteboook(s) from Evernote as ENEX files.
In the Standard Notes application, open the Account menu at the bottom left of the interface, then choose Import.
Click or tap on the Evernote import option, then select your ENEX file(s) for import.
Once the selected file(s) are ready, click or tap on Import to proceed.
Some things to consider:
If you will be importing a large number of notes into the app, or notes with a large number of attachments, it would be best to export your notes in smaller batches of ENEX files, to avoid large syncing payloads that will lead the servers to rate-limit your current connection.
In terms of the individual note sizes within your notebook, the import tool is only able to process notes that are no larger than 3MB.
Notes created using the web clipper on Evernote may contain a number of base64 images, which significantly increase the note sizes in plain text. This can cause the note import to fail due to its size, or it may significantly impact the app's performance down the line. You may save the images on the web clips into a conventional image format (e.g. jpg, png, etc.), then reattach them to the note before exporting it from Evernote.
Due to the variety of Evernote file formats, it has been difficult for us to anticipate all of them. The importer should work well for simple notes, but may struggle with more custom formats. We can't address this issue without your help, so if you ever encounter an Evernote file whose formatting isn't properly converted on Standard Notes, please contact us, and provide the ENEX file you imported.
If the note contains sensitive information, you may remove these from the note, or provide a similar sample note which can help us reproduce the formatting issue.