problem about the preprocess of face image before feed it into face recognition onnx model

I download the resnet 18 pretrained model(R18 | Glint360K | 72.07) for face encoding or face embedding or anything we call it, and it is onnx format. I do not know how to preprocess the aligned face image before feed it into this onnx model. I use the another face detect model and the alignment of dlib library. in the face embedding, I'd like use insightface pretrained model. I'd appeciated if any one can help me.