You should refer a student to a study team when they exhibit consistent academic struggles, behavioral issues, social or emotional concerns, signs of learning disabilities, or health issues affecting their learning. To do this, document your observations, communicate with the student and their parents, complete and submit the necessary referral forms, and follow up by participating in study team meetings to develop and implement a support plan. This ensures the student receives the appropriate assistance to improve their educational experience.
Many school districts will have this process well defined i.e. a flow chart or Google form to fill out etc.
In our district we include a Google Referral Form in Clever so teachers have easy access to the form!

Here is an example of the Google form we use in our district - our multi-disciplinary team created

I agree with Gina. But will add here that data is important and it starts with what you observe in your classroom. Set up a Google or MS file where you can document any behavior or academic concerns that are consistent in your classroom first. Support those concerns with the school work that provides evidence for those concerns. In the State of Florida, once several in-class strategies have been provided students are then taken to MTSS for additional strategies and evaluation before they are advanced for consideration for testing. Also - communication is key with the parents. I always present my concerns by inviting them to meet face-to-face with you where you share your observations. Then ask them what they have observed at home and in previous grades. Parent input is critical in this process.

I collect as much data as I can on a particular student and spend time looking for trends that I notice over time. Once I have a good idea of where a difficulty may lie, I then refer the student to our MTSS team for further help, support, and to arrange support for the student at home as well.

During my entire observation and data collection time, I am trying different strategies with my student to determine if there are strategies that are more effective than others and help the student.