Specify the files scheme in the Project Settings, Files tab.
To extract strings from the .csv or .xlsx files, you should configure them after the upload.
Click Configure across necessary file. To import every cell as a separate string, choose Import all texts option. In other cases, choose Configure columns for import and match the appropriate columns:
- Identifier — the column contains a unique identifier of the source string
- Source string — this column contains the actual text that should be translated
- Source string/translation — Crowdin should consider the texts in this column as the source strings for translation and replace them with the translated texts when downloading the resulting file
- Translation — the resulting translations are placed in this column
- Context — any contextual information that helps to better understand the meaning of the source strings
- Max. length — defines the max. length of the translated texts
In case many of your files have the same scheme, create a configuration template clicking the Templates button. Apply it to newly imported files.
Click Save & Import to proceed.
Multilingual Spreadsheets
If your file contains the columns in which the translations for several languages are stored, check Multilingual Spreadsheet option and map the languages to the appropriate columns. 
Dealing with Column Headers
In order not to translate the headers of the columns, check Do not import the first row (header) option.
Reset Spreadsheet Configuration
Edit button serves to reset the spreadsheet configuration and delete the columns which were manually added.
Configure Several Files at the Same Time
Clicking Configure All in the toolbar to set scheme for several files. Switch between the files to preview either applied configuration is correct. 