How Do You Apply Tax To An ENTIRE Invoice Not Just One Line

New job.  OK, so we're doing something really... quickbooks-challenged here.  Why?  Because we don't know what we're doing.  I'd like to fix this, but there are no clear directions on how to do it.  How do you apply sales tax to EVERY line on your invoice?  Right now, when we create a customer invoice and enter, let's say 10 items, then enter a "Tax" line -- the only thing getting taxed in the 10th line.  All the other 9 items are not being taxed.  So we have literally been calculating the total of the invoice and manually entering in the tax on that line.  It soooo... uhh... quickbooks-challenged.  So, can anyone help... in a simplistic fashion?  How do you apply sales tax to the entire invoice???? Please help 🙏