What is the best time and way to ask professors for TA/GRA positions?

[Quincy_Taft_Coolidge]

After being accepted into multiple Masters of Urban Planning/Public Policy programs for the FA22 term I was curious what was the best way to email professors for TA/GRA positions?

I am completely new to this entire process and have had no prior research experience. 

Although I am going to a professional program and plan on going into the workforce after, I want to eventually get my phD and go into academia, hence why I want some research experience under my belt.