Step 1: Define the function

First, we need to define the function that we will later implement. In JavaScript, this could be done with keyword function followed by the name of this function. In our case, we're going to give the name findAverageNumbersRange, which will receive 2 parameters: start and end.

function findAverageNumbersRange(start, end) {

}

Step 2: Validate the input

The next step is to make sure the input values for start and end are valid. In this case, we need to assure that they're both of type number and that start is less than end.

function findAverageNumbersRange(start, end) {
    if (typeof start !== "number" || typeof end !== "number") {
        return "Both start and end parameters should be numbers";
    }
    if (start >= end) {
        return "Start should be less than end";
    }
}

Step 3: Use Math to find the average

Now that the input values are validated, we get its average by adding start to end and dividing the result by 2. We do this calculation and assign its result to a variable called average.

function findAverageNumbersRange(start, end) {
    if (typeof start !== "number" || typeof end !== "number") {
        return "Both start and end parameters should be numbers";
    }
    if (start >= end) {
        return "Start should be less than end";
    }
    let average = (start + end) / 2;
}

Step 4: Return the average

The final step is to return the average variable. This will give us the result of the operation when we call the function.

function findAverageNumbersRange(start, end) {
    if (typeof start !== "number" || typeof end !== "number") {
        return "Both start and end parameters should be numbers";
    }
    if (start >= end) {
        return "Start should be less than end";
    }
    let average = (start + end) / 2;
    return average;
}

Conclusion

In conclusion, the findAverageNumbersRange JavaScript function will return the average between 2 numbers start and end confirming that start is less than end and both are numbers. If these conditions are not met, the function will return an error message.