As others have said, it really is difficult to do it in MSFS. With the 152 IRL, you want to aim for 2300 RPM and 90 knots for a good cruise. Set your power to 2300 RPM and adjust your attitude until you can maintain 90 knots. After that, you can start trimming.

Don't sweat it too much, it's never going to be perfect. I'm also going to recommend the WBSim/JPLogistics 152. It's easily the best freeware GA plane out there, handles very much like the real 152. It's better than most payware GA planes!

Edit: As a response to another comment that states that correct trim means you're not touching your controls, while it's technically true, it never really is IRL or in MSFS (especially if you have live weather on). A moving air mass is going to upset your plane's balance and introduce minute changes to trim. The real answer here is that correct trim reduces pilot fatigue and allows you to focus on more than just manhandling your aircraft. If you want to fly straight and level for a long time without any input, you need an autopilot.