The instructions here will tell how to run OP-TEE using the MTK1873 device.
Start by following the "Get and build the solution" in the README.md file. When everything has been built and it is booting up, it will stop in fastboot waiting for the binaries. At that point, press reset button and the flashing procedure should begin.
A MediaTek engineer (Howard Chen) has put detailed descriptions on how to connect the device etc in his MTK8173 Getting Started guide. That page also contains information about how to upgrade the firmware etc. There is also a link for how to run OP-TEE. We don't recommend that you follow that, since that guide is very old and has been superseded by this particular guide.