1. When you receive an email or phone call from your DSO (or any other agreement contact methods), go and pick up your new STEM OPT Extension endorsed I-20
Sign the new I-20 at the bottom of page 1.
Photocopy after signing; you will send the photocopy, not the original I-20
Copy of Form I-20 with your signature and STEM Extension endorsement by your DSO (must arrive at USCIS within 60 days of issuance)
Form I-765 (Be sure to include your employer's name as listed in E-Verify and E-Verify Company Identification Number or valid E-Verify Client Company Identification Number)
Copy of your transcript
A copy of Form I-94 (which can be obtained at https://i94.cbp.dhs.gov)
A copy of your current EAD card (front and back)
2 passport-style photos. Lightly write your name and I-94 number on the back of each photo. (Review number 2. C on page 8 of the "Required Documentation" section of USCIS photo instructions
https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/photos.html.
Form I-765 filing fee (extra 85 biometric services fee is not required for OPT applications) paid by money order, personal check, cashier's check, or credit card using Form G-1450, Authorization for Credit Card Transactions. If you pay by check, you must make your check payable to the U.S. Department of Homeland Security.
Completed G-1145 Form (optional; it signs you up for email/text message notification of your application's arrival at USCIS)
Note: the I-983 Training Plan does not need to be submitted to USCIS (unless USCIS requests a copy during STEM OPT extension processing). You should keep a copy of the plan for your own records, and your DSO also keeps a copy.
Also, USCIS I-765 filing instructions (both general and STEM OPT) do not require copies of previous I-20s, passport biographical page, nor F-1 visa page. You may include these copies if you want to, but they are not required.