Look up business model canvas. Identify all the core components. See which parts are riskiest. Very often the risk lies in marketing channels, value proposition, and partners.
Validation is testing those risks. If your risk is in finding partners, find those partners first and talk to them, validate that this is something they want. You don't need a product for this; sometimes slides are good enough.
If you're testing marketing channels, you can create a dummy page, market that through your channels, then shut it down. Don't do a landing page. Nobody trusts landing pages.
If you want to test value proposition, build your product against that, no more. If the demand is strong enough, nobody cares how hacky it looks, which is often why enterprise software is so ugly.