If you want to hack this plugin, you need to have a working Golang build enviornment (see the Golang website) to get started.
Terraform has its own mechanism for enabling traces, as detailed here.
This plugin logs to stdout, and its outputs are collected by terrafiorm and interleaved with those of the internal engine.
To enable logging, define the TF_LOG and optionally the TF_LOG_PATH environment variables before running the command, e.g.:
$> TF_LOG=TRACE TF_LOG_PATH=./terraform.log terraform plan