Folks often talk about disciplinary action like it’s some kind of rocket surgery.  I don’t think that it is.

It’s really just a fancy term for the following process:

Clearly communicate expectations.
Clearly communicate that expectations are not being met.
Clearly outline the consequences of continuing failure to meet expectations.
Offer guidance, support, mentorship, coaching, and direction.
Reevaluate and follow through on consequences.

Simple.  Make sure they know what they need to do, make sure they know that they’re not doing it, make sure they know what’s going to happen if they don’t, do everything you can to help them succeed, and then hold them accountable.

There is no need to be aggressive, rude, or unprofessional about this.  It is black and white, plain and simple.