Just follow the instructions below. You will need a Google/Gmail account. (make one here)
There is currently NO COST to use the Gemini model for 20 requests per day. See details here

1. Go to Google AI Studio
2. Click the Get API key link in the lower left
3. Click the Create API key button at the upper right
4. In the dialog that pops up, click the Choose an imported project dropdown and select Create Project. Then press the Create project button and wait while Google creates your project
5. Once the project is generated, press the Create key button to create a new key.
6. Press the Copy button on the row with your key to copy your key, then in Ask Steve go to the Models page and press Add New Model.
7. Select a Google Gemini model, paste in your API Key and then press Save New Model.
8. That's it! You get 20 requests per day for free with certain models, and can see your rate limits here. If you'd like more requests per day or want to use different models, you will need to Set up billing in AI Studio.