Need help My mac has both “contacts” app and “Libre Office” with spreadsheet and database and other stuff.
Please tell me where do I manually input 500 clients names, address etc (no none of it is in any kind of digital form anywhere all info was hand written addresses from a convention it all needs to be put in manually ). I need to create a mailing list that can be sent to a mail house in the form they want so they can print the names and addresses on advertising postcards etc. They want it to be in excel I dont’ have excel I have a mac with Libre Office and a contacts book. Help please 
Good job for LibreOffice Calc (except for having to manually type in all that info
Open a new spreadsheet, use the first row to set up the various address field columns - such as first name, last name, address, city, state, zip code. You can make these columns as wide as needed to see the information being input. Then just have a great time typing in all the info. (Don’t forget to save your work on a regular basis.)
To save the file as something a mailing service might want - you said Excel - do a “save as” and select “Microsoft Excel 97-2003 (.xls)” or “Microsoft Excel 2007-2013 XML (.xlsx)”. (The LibreOffice default is ODS Spreadsheet (.ods)).
Some services may want a “comma delimited” file that is also available in the Save As dialog as Text CSV.
You could also do a mailing label print yourself using a LibreOffice label template and the mail merge capability.
Good Luck.