How to...>>>>

http://www.ehow.com/how_4477902_use-mixture-control-airplane.html

Paul....G-BPLF...FS 2005...FS Nav...and a Mixture Knob... ...!

That article describes what a mixture adjustment accomplishes, and that's about it..not a very good "lesson".

You'll have to take your lashings, for flying a complex twin, before fully understanding how a pilot controls mixture in a fixed-prop single 

First, to answer one question..  you control your airspeed by pitch. File that away as gospel truth, never to be questioned 

Now.. for a fixed-prop single, leaning during the climb:  In a perfect world, you'd always have a perfectly functioning EGT. As fuel enters a combustion chamber, it actually acts as a coolant first,  and then combusts. As you lean a mixture (I start at 3000msl), the exhaust gasses will get hotter.. then peak, and then further leaning gets into scenario where exhaut gasses cool, because so much less energy being generated. Somewhere near that peak, is where the mixture is optimal, and the most HP is being produced. A long-time practice for the best compromise between; engine life, economy and power; is to run a tad on the rich side of that peak (ROP). So.. if you have a responsive, accurate EGT, you'd lean till the gauge peaked, and then richen a couple notches... remember that gauge indication (most have an adjustable reference line), and try to keep it there, throughout the climb.

Once leveled and trimmed.. you can lean for cruise. It's OK (and desirable) to go lean of peak (LOP) for a cruise. Just remember to go back to a climb EGT indication, if you end up climbing again. Descents will be at lower power settings, so you kinda best guess per your altitude, and re-lean for cruise at your new altitude. If it's a descent for landing, just go full rich at 3000msl.

In the real world, I don't trust EGT gauges on rental aircraft, so I "wing it". Start leaning carefully at 3000msl, giving yourself margin for error on the rich side. After enough flying, you'll feel a sweet-spot for a climb. For cruise, lean slowly until the engine "complains", and then richen a tad. That's usually a bit LOP.

During all of this, you control your airspeed by pitching for a desried climb-speed  (Vy at first, and then cruise-climb airspeed) 

Now.. for a constant-speed prop, it's a bit different. The article mentions using RPM for reference, but for that to even be considered, you would have had to let the mixture get way out of whack. An attentive pilot will never have to make mixture adjustments that will effect RPM, because sans substantial power changes (via poor mixture), a constant-speed prop won't LET the RPMs change. And manifold pressure is a function of RPMs and throtle position, so it shouldn't change either. You have to hope that an EGT in a high-performance airplane is in good working order. That would be your primary reference for leaning. Also, you'll know what fuel consumption numbers will occur for different power settings, at different altitudes..so you can incorporate the fuel-flow gauges. Between those two references, you should be able to properly manage mixture.

And again, you'll be managing your airspeed by pitch 

Side note: Many fixed-prop, fuel-injected singles have fuel-flow gauges.. you can reference them too.

The MSFS dilema:  MSFS EGT gauges are slow to react, and don't react enough to be useful. And there's a quirk in the MSFS engine algorithm that makes fuel-flow gauges behave oddly. MSFS's method for punishing poor mixture (reduce power), is to restrict fuel-flow proportionally to "wrongness", either side of optimal. This weird twist gives you the world's best mixture indicator. Just lean to keep fuel-flow at a peak.. and then go slight less than peak for cruise.

You can witness this bizarre-ness by climbing to 6000msl without touching the mixture. Your initial leaning will actual cause fuel-flow to INCREASE.. then peak, and then decrease. It kinda acts like an EGT.

I've even re-bitmapped a fuel-flow gauge and used it as an EGT ... 

One little technique that works a charm.  Switch to a single mag, lean to max rpm on that one.  When you switch back to both, it'll be just right.  Of course, if you have a panel full of exotic stuff, might as well use it  .