I have tried Several methods to set a field using it's id to the value of my script or calculation and it just breaks everything.

I have tried

fieldID.value

I have tried

var = value;
fieldID == var;

FieldID == value;

document.getElementById("fieldID").value = "myValue";

All at the Field level not the overall javascript level. My point being there are a LOT of ways to set a value. (with these I can't even make the number 1 show up in a read only text field) All of these Validated and then either made the form not work at all or the field was undefined.

So I ask for a tutorial that specifical helping people know what method works best for inputting data into the fields of your form.