A phone is not exactly similar a mouse or keyboard, so many are surely skeptical, but it's actually a pretty good option, if you have no mouse available.
It's pretty simple to use Remote Mouse by installing an app on your phone and an application on the PC you want to control. The phone will then automatically connect to the PC, assuming that both devices are connected to a wireless network. Run the Remote Mouse app on your phone, and scroll to the Start button. You can scroll with your finger from right to left on the phone screen.
Click the Start button that pops up on your phone, and you will then find the name of your PC. Click the name, and then you get a screen on the phone with various functions. From there you just need to tap with your finger on the various functions. For example, touch the keyboard icon on your phone to activate the phone as a keyboard, or tap on the application icon to get a list of installed software.
You can also move your finger across the phone screen, and tap the touch the screen once or twice to perform left or right clicks, or you can scroll with two fingers. You can launch a variety of software via the phone that are related to programs pinned to the taskbar in Windows.
Some features require you to buy Remote Mouse, for them to act via the phone. This includes control of audio and video.
If you want to protect your computer against attacks via this feature, you can choose to password protect access. Right-click the Remote Mouse icon on the taskbar at the bottom of Windows, and choose Preferences and Settings, and Set password for your computer, and finish with Apply.
Remote Mouse is available for free download from the official website. The app is available for Windows Phone, iOS and Android), and requires that you also install the program on your PC.