Let's cover a few basics.
The strength, but also the weakness, of OpenTX is its flexibility. Because you can almost do anything, a lot is dependent on what you did before. So what you have to start with may be very different that what someone else used as a base, making it difficult to only take parts of what they did. Better to learn how the parts work together, so you can tune it to your particular setup.
So, some basics:
1. All sticks/switches/knobs/sliders on the radio are generically referred to as "Sources". They all send values ranging from -100 to +100 to the rest of the system. Switches only send two or three (-100,0,+100 or -100 and +100). The rest are variable for the whole range, depending on their position.
2, INPUTS is a list that allows you to create a
new Source, based on one of the above.
a. If you do not need to change the behavior of a stick/switch/knob/slider at all, it does not have to be listed here. It can be, but is not required.
b. It is just a numbered list. Within the numbering, it does not matter what order the items are listed. Meaning, as an example, "Ail" does not have to be first, "Ele" second, etc. You can put them anywhere on the list you like.
c. For any one number on the list, you can create multiple lines to have different behaviors for the base Source - two different rates, for example.
d. So, as an example, if your Aileron stick moves the Aileron servo too much, you create an INPUT that tells it to cut the throw to 70% of normal. You can add a second line to also tell it to cut things by 50%. Which line to use would then also have a switch defined, so you can select it while flying.
e. Note that you now have two Aileron Sources! The original stick, and the one listed under INPUTS. One needs to pay attention to which one is used, later in Mixing. They may appear to have only slightly different names or symbols used in later lists.
3. MIXING - very basic
a. This links a source from above to a channel on your receiver. Again, basic order doesn't matter, as long as you plug the correct servo into the correct slot on your receiver. If Elevator is on the list for #1, then the Elevator servo must be plugged into the first channel slot on the receiver. Some receivers do not number their slots, but use letters for the default control. If yours has the first one labeled "A" and you want to connect the aileron servo there, then in Mixing you should list Ail in the #1 position. (but if you have already defined Elevator for #1 in mixing, as long as you plug the elevator servo into the "A" (first) slot, it will still work!)
b. Make sure you are using the source you want. The original Source, or the modified one defined in the INPUTS list.
c. Generally, for basic controls, do not change the amount of movement (the "weight") without good reason. Leave at 100%. If you need more or less, change it in INPUTS. Not wrong if you do, but can make things confusing if things don't work like you think they should.
4. MIXING - true mixing (we'll assume you have the above all set and working for the basic channels)
a, By defining multiple lines for one channel, you can use two different Sources to move one servo. We'll use Ail to Rudder, like might be done on a Piper Cub, as an example.
b. The description is "When I move the aileron stick, I want the rudder to also move a little bit".
c. Since it is the Rudder to be moved, go to the Rudder channel line in the MIXER. Add a second line, define it to use as a Source the Aileron control (probably the one you defined in Inputs, so that this also uses the rates defined there, rather than the base Aileron stick).
d. Since the Ail was defined to be cut down to 70% (or 50%) in INPUTS in our example, now moving the stick will do the same for both Aileron AND Rudder!
e. If this is too much rudder movement when the Ail stick is used, you can reduce the "weight" on that mixer line to give the further reduction you want. This is one of those good reasons to change the weight.
f. On that same line, you can define a switch position if you want, to be able to turn it on or off in flight.
Of course there are many more options that allow you to highly modify things. But once you grasp the basic framework of how things effect each other, you can start to build from basic mixing, to very complex setups.