Depending of your module tools, a direct method you can take to flash any firmware to your module is has shown below;
1. A USB to TTL/Serial adaptor/converter is needed and you need to download and install the device of the one you have. You can download driver for any of the USB to TTL converter here.
2. F-M/F-F jumper wire.
3. SIM card
4. GSM/ESP module
5. Flash download tools
You can then follow connection for any of the module that you have;
Note: To connect for SIM900A or equivalent, I recommend to use the 5VT and 5VR pins.
For example to flash a SIM900 module now;
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