Can someone guide me through the process of how to link an existing expense to an manual imported bank transaction?

Yes, I'm here to guide you through matching your existing expenses to the imported bank transactions, @britt5. 

Here are a few reasons the system didn't recognize any possible matches: 

• The expenses were manually cleared in the register. 
• The amounts in QuickBooks Online (QBO) are different from the imported entries. 
• Transaction types are different. 
• The transactions are older than 180 days. 
• The expenses were recorded in different bank accounts. 

First, let's go to your register to check if the expenses were already cleared. From there, you can review them if they have the same amounts and transaction types with the imported ones. Also, you can verify their dates. I'll guide you how. 

1. Go to Accounting from the left menu.
2. Select Chart of Accounts.
3. Locate the bank account you want to review.
4. Hit View register from the Action column. 
5. Click the Filter icon. 
6. Choose Expense as the Transaction Type. 
7. Hit Apply. 
8. Locate your expenses and make sure their status are not cleared "C" in the Reconciliation column. If necessary, click the boxes to make the fields blank.
9. Compare the amounts, dates, and their transaction types with the imported entries. If necessary, change the details to match the imported ones. 

The screenshot below shows you the last five steps. 

After that, let's make sure the expenses are associated with the correct bank account by manually opening each expense. Once verified, go back to the For Review tab and refresh the page. This way, you can check the transactions if there are suggested matches in the program. 

However, if there aren't any matches, you'll have to select the Find Match tool to resolve them manually. Then, use either the Show or Search field to locate the expenses. View the screenshot below for your visual reference. For more information, see Tip #3 through this article: Tips for Working in QuickBooks Online’s Bank Feed: Matching Transactions. Once done, click Save to make sure you have no duplicates in the software. 

In case you want to make any changes after you matched the transactions, you can undo them from the Reviewed tab. These will go back to the For Review tab. From there, edit and categorize them properly. To learn more about this process, check out this article: Unmatch or Move Bank Transactions After Reviewing Them.

Moreover, it's a great idea to reconcile your account monthly. Doing so will ensure the account in QBO matches your bank statement. For more details, visit this article: Reconciling An Account. 

I'm just a comment away if you need further assistance. Enjoy the rest of your day,  @britt5.