First try

One per line, "key: value" — e.g. salutation: Mr. — usable as {{salutation}} in campaigns

Columns in order: email, name, tags, custom fields. Only email is required. Loads the file's contents below so you can review before importing.

One per line: email or email,name or email,name,tag1|tag2 or email,name,tag1|tag2,field1:val1;field2:val2