Missed RMD

Hello I erroneously missed my RMD in 2021.  I will will fill out form 5329 for missed Rmd and submit after I file the 2021 taxes with turbo tax.  Do I send check for estimated tax on rmd (20% historically) ? I don’t see where my broker vanguard offers to hold back tax.  I will fill out the reasonable cause details and send note so I don’t have to pay 50% penalty. I would submit with my taxes but I believe this would not allow me to electronically file.  I have read that I should do this as a separate transaction in the mail. Please help! 
thanks

joe