You don't actually "control" the mechs if you want to be specific. You issue focus fire and movement orders (which can be very detailed or just a "go here") and your pilots attempt to carry them out. There's a "freedom" slider for how much leeway they have to vary from your orders, and if they panic they'll just flat out ignore them and do something else like fire wildly or run away.

The amount of control (and mental effort) you choose to put in is entirely up to you. Micromanaging the whole team one at a time can help in some cases, other times (many to most) its just select-all and tell them to get a move-on. If you aren't keeping an eye on them they WILL do something dumb and get themselves killed and the mech trashed. Everything can and will go wrong when out on missions if you aren't situationally aware. Some of the above is mitigated as morale, trust and competence go up, but it never really goes away. Just something you have to cope with, simulations of people actually running around in your mechs doing hopefully-good-enough jobs to not die and complete the mission. ^^

Examples of some more advanced ordering.
-Orders given while paused don't count towards your order delay limit till you unpause, so you can refine them.
-Drawing movement paths into walls with a mech who has a drill will make them mine out that portion of the wall to clear a path (or open firing lines).
-That freedom slider I mentioned is in the bottom left during missions (when you aren't in full screen). 0% they try to adhere to exactly what you tell them, 100% I swear they just wander off and do whatever. 15-20% seems to let them have enough wiggle room to open fire lines (your mechs block eachothers' line of sight) and dodge closing enemies while still staying on-mission.
-Your mechs will normally face the direction they're going or the nearest enemy, you can get them to be looking where you want when they're not busy by putting your mouse in the direction you want them to be looking, useful for if enemies are about to come around a corner or exit a wall and none of your idiots are looking that way. This only works when relatively close to them (slightly more than flashlight range?).

Ok, thank you very much for the detailed answer.