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GUIDE_SERVICE_ACCOUNT.md

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+# Google Service Account Setup for Google Sheets Rate Assistant
+
+This guide explains how to create a Google service account, enable the Sheets API, and configure the project to use service account credentials.
+
+## 1. Enable the Google Sheets API
+
+1. Open the Google Cloud Console: https://console.cloud.google.com/
+2. Select an existing project or create a new one.
+3. In the left menu, open **APIs & Services > Library**.
+4. Search for **Google Sheets API** and click it.
+5. Click **Enable**.
+
+## 2. Create a service account
+
+1. In the Cloud Console, open **IAM & Admin > Service Accounts**.
+2. Click **Create Service Account**.
+3. Enter a name and description.
+4. Click **Create and continue**.
+5. Skip granting optional roles or add a minimal role if required.
+6. Click **Done**.
+
+## 3. Create and download credentials
+
+1. Find the service account you created in the list.
+2. Click the service account name.
+3. Open the **Keys** tab.
+4. Click **Add Key > Create new key**.
+5. Choose **JSON** and click **Create**.
+6. Save the downloaded JSON file securely in your project folder, for example `credentials.json`.
+
+## 4. Share your Google Sheets with the service account
+
+1. Open the Google Sheet you want to export.
+2. Click **Share**.
+3. Add the service account email address, which looks like `...@...iam.gserviceaccount.com`.
+4. Give it **Viewer** access.
+
+## 5. Configure the project
+
+In `config.json`, set the `credentialsPath` field to the JSON file path, and remove any `apiKey` or `apiKeys` entries if you are using service account auth.
+
+Example:
+
+```json
+{
+  "credentialsPath": "credentials.json",
+  "rateLimitPerMinute": 50,
+  "documents": [
+    {
+      "documentId": "YOUR_SPREADSHEET_ID",
+      "outputDir": "output/your-document",
+      "sheets": [
+        {
+          "name": "Sheet1",
+          "outputFilename": "sheet1.csv"
+        }
+      ]
+    }
+  ]
+}
+```