Flying and autolanding with FSX ATC (so no complete plan in FMC)

I remember seeing a topic on this forum about how to fly the 737NGX without having a complete flightplan in the FMC. I mean, I'd like to fly using the FSX ATC (even though I know it *****), but that means I will be vectored around and so I can't put a plan in the CDU and hence the FMC can't program the descent etc. for me. So... how do I know which speeds I have to enter manually on the MCP...? And how do I autoland using the MCP only? Or isn't that possible and do I HAVE to enter all that data in the CDU...? It's been a while since I flew airliners as complex as this one, so I could use some help but I can't find that topic anymore...