Hi Teahconnect,
From the scool of hard import knocks, whther you use Method import feature or QB, to save yourself grief make sure that your names and addresses do not exceed expected field lengths. I don't have them at my fingertips but if you google "field length for QuickBooks" you can find them. I set up fields in Excel to do this. And when importing to Customer table, there is only one contact per customer. (QB will only allow one name) If multiple contacts then need to upload to Contacts table referencing the customer.
If you intend to use any customer custom fields in QB and are uploading, then set those up in QB first. Sync over. This makes them available in Method. Then you can import using those names if you upload thru Method. If using custom fields in Method only, then you can generate these during the import process.
Considering these things done before you upload will make the experience much more pleasant and save you lots of time.
Hope this helps...Look at help center as Justin's suggested also.