How do I re-characterize the excess 2018 contributions that I made to my IRA?

You can't recharacterize the excess contributions, but you can correct them before 04/15.  First, you need to remove the contributions from the IRA 04/15.  Contact the plan custodian and let them know you need to remove excess contributions.  

TurboTax will prepare Form 5329 to report the excess contribution unless you take the necessary steps to resolve the excess and indicate to TurboTax that you have done so.  If you obtain a return of excess contribution and indicate this to TurboTax, TurboTax will prompt you to prepare the necessary explanation statement with regard to the return of excess contribution, and no penalties will be assessed.