If you are trying to put your own website behind WebRender, you are likely looking for the Edge Cache API and not the Instant Render API.
If you don't care about cache and just want to render pages, create an API deployment an render away. Just append the URL you want to render to the deployment URL.
First, make sure to Create an account and create an API deployment (should look like it does below).
If you're used to other pre-render services, this will work in a very similar way. In fact, you can likely exchange them one by one. The only difference is that WebRender is a reverse proxy, which means we can give you your own domain instead of having to worry about headers and API keys.
https://YOUR_DEPLOYMENT.webrender.io/https://reddit.com will give you today's reddit page, fully rendered.
You'll really only want to use this if you are rendering other people's websites (eg if you are a search engine or a crawler bot). Use the Edge Cache API for every other scenario since it will be the fastest, and it has examples on how to get started using various existing web configurations.
This is the configuration if you are deploying via the API
You will be billed the same way a regular deployment is billed if you use the Instant Render API (by unique pages rendered) - keep your deployment ID private so others can not use your deployment and incur charges. Reach out if you have any issues.