We advise the use of Mozilla Firefox for editing pages. You don't have to follow this advice, and other browsers like Chrome or Safari, or even Microsoft Edge, will work perfectly well. Since Firefox has a builtin spell checker and you likely have already preloaded it with words you use, it makes sense to use this in Mara, though.
The other advantage of Firefox is that it provides for image resizing by way of corner handles, which is incredibly useful, but hard to achieve in other browsers.
CKEditor also has its own spell checker, which is turned off as supplied since having two running would not be good. If you'd rather use the CK spellcheck then it's possible to turn that on in ck_config.js.
With Mara in edit mode, Firefox should automatically start spellchecking, and underline in red any words it doesn't recognise. If this doesn't happen you probably need to go into the Firefox settings and turn spell checking on. You will also need a dictionary to suit your locale, and you can get these from mozilla.org.
With the caret on a red-underlined word, hold down the Control key and right-click the underlined word. You will then have an option of choosing a suggested replacement, or of adding the word to the dictionary.
Although, it suggests I change 'Firefox' to 'Firebox.' Hmmm. I think maybe the Mozilla guys haven't noticed that one yet. ;)
If you would prefer to have grammar checking as well, there are plugins available for Firefox.