Is there a shortcut to those letters?

Assuming you are using Windows then install the United States-International keyboard layout.

See below for instructions for Windows 7.

You can then use " followed by one of e, y, u, i, o, or a to produce ë, ÿ, ü, ï, ö, or ä

How to use the United States-International keyboard layout

Adding the United States-International keyboard layout

To add the United States-International keyboard layout, follow these steps:

• Click Start Start button, type intl.cpl in the Start Search box, and then press ENTER.
• On the Keyboards and Language tab, click Change keyboards.
• Click Add.
• Expand the language that you want. For example, English (United States).
• Expand Keyboard list, select the United States-International check box, and then click OK.
• In the Default input language list, click Language name - United States-International (where Language name is the language that you selected in step 4), and then click OK two times.

• In the Regional and Language Options dialog box, click OK.

  Notice that the Language bar appears on the taskbar. When you position the mouse pointer over it, a ToolTip appears that describes the active keyboard layout.

• Click the Language bar, and then click United States-International on the shortcut menu that appears.

  The United States-International keyboard layout is selected.

Creating international characters

When you press the APOSTROPHE ( ' ) key, QUOTATION MARK ( " ) key, ACCENT GRAVE ( ` ) key, TILDE ( ~ ) key, or ACCENT CIRCUMFLEX,. also called the CARET key, ( ^ ) key, nothing is displayed on the screen until you press a second key:

• If you press one of the letters designated as eligible to receive an accent mark, the accented version of the letter appears.
• If you press the key of a character that is not eligible to receive an accent mark, two separate characters appear.
• If you press the space bar, the symbol (apostrophe, quotation mark, accent grave, tilde, accent circumflex or caret) is displayed by itself.

The following table shows the keyboard combinations that you can use to create the desired character.

How to use the United States-International keyboard layout

The layout doesn't really matter.

Here's how to add the International Keyboard:

1. Open Settings (Start > Settings)
2. Click on Time & Language on the sidebar.
3. Click Language & Region.
4. Select the dots next to your current language, and click Language Options in the dropdown.
5. Scroll down to Keyboards, and click "Add a Keyboard"
6. Scroll down the list until you see "United States-International" and click it.

There will be an option in the taskbar that looks like this:

(Depending on your region, there might be something different)

Click it, then scroll down to the United States-International option and click it.

For reference, here's the layout:

You will have to use SHIFT+ Character for the upper one. (Example: SHIFT + ` , then "n" would produce ñ.)

Credit: (https://miamioh.edu/cas/centers-institutes/interactive-language-resource-center/tech-support/index.html)