Keep the model in T-pose. When merging select your model then shift + right click to select the smash model then press ctrl + j. After that go into edit mode, select the vertices of the smash model only and delete them, then press A to select all of the vertices of your model and just move them to where the smash model was.
Please keep in mind that this is not easy, it's doable but takes practice. I'm not here to hold your hand through the entire model importing process. Use the tutorial, READ EVERY SINGLE WORD OF IT AND DO EVERY SINGLE STEP IN ORDER.