How to pass arguments to class methods and return values to the main method?

I found a random assignment online and am using it as practice to try and improve as a coder, but I am having a lot of difficulty with this one area of the assignment. It wants me to "do the calculations and conversions inside separate methods. Pass the needed information into the method from the main method. After calculation is done, return the result into the main method for printing." I have been working on this for a long time now, and if I don't complete this it's really going to bug me, so any help is greatly appreciated. I apologize if this is a stupid question, I am very new to coding.

Here is my code:

import java.util.Scanner; // Allows for use of scanner in class

public class MeasurementConversion // Start of class
{
    public static void main(String[]args) // Start of main
    {
        Scanner readConsole = new Scanner(System.in); // This is the scanner
        String convert;
        double poundAmount; // These are the doubles that store the measurements
        double kilogAmount;
        double ounceAmount;
        double gramAmount;
        double feetAmount;
        double meterAmount;
        double mileAmount;
        double kilomAmount;

        System.out.println("Hello, I am a conversion calculator. I can convert pounds to kilograms, ounces to grams,"); // This is where the scanner begins prompting the user for input
        System.out.println("feet to meters, miles to kilometers, and vice versa.");
        System.out.println("What would you like to convert?");
        convert = readConsole.nextLine();
        if (convert.equalsIgnoreCase("pounds to kilograms")) // This converts from pounds to kilograms
        {
            System.out.println("Ok, what is the amount of pounds you know?"); 
            poundAmount = readConsole.nextDouble();
            kilogAmount = poundAmount * .4536;
            System.out.println("That is " + kilogAmount + " kilograms.");
        } // End of if statement
        if (convert.equalsIgnoreCase("kilograms to pounds")) // This converts from kilograms to pounds
        {
            System.out.println("Ok, what is the amount of kilograms you know?");
            kilogAmount = readConsole.nextDouble();
            poundAmount = kilogAmount * 2.20462;
            System.out.println("That is " + poundAmount + " pounds.");
        } // End of if statement
        if (convert.equalsIgnoreCase("ounces to grams")) // This converts from ounces to grams
        {
            System.out.println("Ok, what is the amount of ounces you know?");
            ounceAmount = readConsole.nextDouble();
            gramAmount = ounceAmount * 28.5;
            System.out.println("That is " + gramAmount + " grams.");    
        } // End of if statement
        if (convert.equalsIgnoreCase("grams to ounces")) // This converts from grams to ounces
        {
            System.out.println("Ok, what is the amount of grams you know?");
            gramAmount = readConsole.nextDouble();
            ounceAmount = gramAmount * .035274;
            System.out.println("That is " + ounceAmount + " ounces.");
        } // End of if statement
        if (convert.equalsIgnoreCase("feet to meters")) // This converts from feet to meters
        {
            System.out.println("Ok, what is the amount of feet you know?");
            feetAmount = readConsole.nextDouble();
            meterAmount = feetAmount * .3048;
            System.out.println("That is " + meterAmount + " meters.");
        } // End of if statement
        if (convert.equalsIgnoreCase("meters to feet")) // This converts from meters to feet
        {
            System.out.println("Ok, what is the amount of meters you know?");
            meterAmount = readConsole.nextDouble();
            feetAmount = meterAmount * 3.28084;
            System.out.println("That is " + feetAmount + " feet.");
        } // End of if statement
        if (convert.equalsIgnoreCase("miles to kilometers")) // This converts from miles to kilometers 
        {
            System.out.println("Ok, what is the amount of miles you know?");
            mileAmount = readConsole.nextDouble();
            kilomAmount = mileAmount * 1.61;
            System.out.println("That is " + kilomAmount + " kilometers.");
        } // End of if statement
        if (convert.equalsIgnoreCase("kilometers to miles")) // This converts from kilometers to miles
        {
            System.out.println("Ok, what is the amount of kilometers you know?");
            kilomAmount = readConsole.nextDouble();
            mileAmount = kilomAmount * .621371;
            System.out.println("That is " + mileAmount + " miles.");
        } // End of if statement
    } // End of main
} // End of class