Defining Variables for use in multiple methods

Instead of declaring (e.g.: double euro;) inside the method, you can declare it as a class member:

static double euro;

Outside of the method, at the class level.

This should work for both euro and tC, but I need to advise that this is not the best way to use java, as it is an Object-Oriented language. But I'm sure you'll get there :).

In the readDouble() method you are returning a double value but you are not getting back that value anywhere in your performTemperatureConversion() and performCurrencyConversion() methods. If you want to use the same value throughout the class declare a static variable and get the scanner value in it instead of using the local variable in methods. Pass the same value in methods degreeConversion() and currencyConversion().

You need your numDouble variable to be a Class Member Variable.

This means that any method in your class can "see" it (i.e. can use the value or update the value.

To do this just define the variable outside of the methods, but still within the class itself.

By convention, and in most cases, you'll want this to be a private member variable, so you'll want to do something like this:

// Defined here as a private member variable
private double numdouble;

public static double readDouble()
{
  // numDouble definition removed here, no need.
  Scanner input = new Scanner(System.in); 
  System.out.print("Enter a double number: ");
  numDouble = input.nextDouble();
  return numDouble;
}

Then you just need to make sure that this readDouble() is called before numDouble is used.

As I understand you want to be numDouble and tC two variables visible in all of those methods. Obviously you use the keyword "double" more like a variable import similar to the keyword "global" in PHP.

But your code does in fact declare the variable method local and throw it away at the end of the method. This way you can't share the variables over several method calls. The simplest way to fix this is to define two static methods at class level:

package stackoverflow;

import java.util.Scanner;

public class MethodsMS {

private static double numDouble, tC, euro;

// required main method of the class
public static void main(String[] args) {

    performTemperatureConversion();
    System.out.println(); // blank line
    performCurrencyConversion();
} // end main

// (no changes needed above this point)
// --------------------------------------------------------------  
// (write the following methods)

// --------------------------------------------------------------

// write method to perform temperature conversion...
public static void performTemperatureConversion() {
    readDouble();
    degreeConversion();
    display();
}

// --------------------------------------------------------------

// write method to perform currency conversion...
public static void performCurrencyConversion() {
    readDouble();
    currencyConversion();
    display();
}

// --------------------------------------------------------------

// write method to obtain an input double value (use Scanner)...
public static double readDouble() {
    try (Scanner input = new Scanner(System.in)) {
        System.out.print("Enter a double number: ");
        numDouble = input.nextDouble();
    }

    return numDouble;

}

// --------------------------------------------------------------

// write method to print an input string inside a formatted box...
public static void display() {
    System.out.print("Conversion number is now: " + numDouble);
    System.out.print("tC is now: " + tC);
    System.out.print("Euro is now: " + euro);
}

// --------------------------------------------------------------

// write method to convert F to C...
public static void degreeConversion() {
    tC = (numDouble - 32) * 5 / 9;
}

// --------------------------------------------------------------

// write method to convert $ to Euros...
public static void currencyConversion() {
    euro = numDouble * 0.894614;
}

} // end class

BTW: You also forgot to close the input variable. I fixed this.