How to put the upcoming experience in Grad School Application?
The best way is probably to get one of you letter writers to mention it "K.S. will TA for my course in ML this coming term." Something simple. This shows you are both trusted and respected. 

You can mention it in a simple way in a SoP or a CV. 

Don't overdo it. If you make it sound like your biggest accomplishment you will probably not benefit. 

The normal decision process is based on other things. Make sure you focus on those.