They’re both available online for free and they both have a Premium version (not free and with more useful features).
If you want to use SpellCheckPlus you just need to copy your text and paste it. This tool will give you a feedback for possible corrections (it’s intended to be a teaching tool).
This is how it will appear you when you’ll copy your text:
Grammarly is also easy to use and it’s higher in accuracy. Furthermore, it is the only online grammar check service that has a tool to check for plagiarism.
You can add it to Chrome if you want to be always protected from spelling or grammar mistakes while writing on the web.
It will appear like this while you write (you can see the icon in the right corner):
If you make errors, it changes:
By clicking on the icon, this will appear on the screen: