All you need to do is read Rorie and Uncle Gordon with a Scottish brogue! Which, of course, is much harder than it sounds--or as hard as it sounds. Stevenson was noted for his great facility with writing in the sounds of a Scottish dialect. After a while, once you break the code and have a good dictionary on hand--which for the task of decoding Rorie and Gordon would be the on-line The Free Dictionary by Farlex for it has many dialectical Scottish words like "claver" and "skreigh"--you will pick up their brogue or at least be able to decipher it.
Let's get an overview of their dialect using Rorie first because he says least! Rorie is, of course, Uncle Gordon's aged serving man, so one might expect his dialectical speech to be simple, unembellished, restrained. And that it 'tis, that it 'tis. Rorie's first remark is, "It will be hard to cheesel." A glance at The Free Dictionary on-line shows that this is not a Scottish word, or not one listed.
If we start from the first conjecture, that it's not a Scottish word, we draw the conclusion that it is an English word written phonetically by Stevenson according to dialectical pronunciation. So what in English sounds like cheesle? The word chisel sounds about like it. Does chisel make sense in context? (1) They are talking about the new foreign wood that the boat was repaired with. (2) Rorie answers reluctantly. (3) Wood has to be chiseled. So yes, a phonetic spelling of chisel does make sense in context. Rorie is preoccupied and not conversant, so instead of entering the conversation he responds with a negative comment about the wood being hard to work with. Decoding Rule No. 1: consider phonetic spellings of English words.
Let's look at Uncle Gordon now since he says the most. We'll use these random quotes:
**But, man! the dunt that she cam doon wi’ when she struck! Lord save us a’! but it’s an unco life to be a sailor—a cauld, wanchancy life.
**there’s nane that’s like the sea deils
Look at "cam doon." Here you have two more phonetic spellings: came and down. Look at "nane." Here is a representative of Decoding Rule No. 2. Scottish words like nane, gane and stane are the variable spellings of the English words none, gone and stone. Now look at dunt, unco, deils, wanchancy, claver and cauld. These you will find in The Free Dictionary. Dunt = thump. Unco = uncanny, surprisingly unusual. Deils = devils. Wanchancy = unlucky; dangerous and risky. Claver = gossip or talk idly. Cauld = cold. And there you have Decoding Rule No. 3: Check a dictionary that has Scottish dialect words in it.
Once you get accustomed to words like nae (no; not), sic (such) and cauld and nane, gane, stane, and once you get accustomed to recognizing phonetically spelled English words, you'll get along much more easily in between words that you'll have no choice but to look up: Who could possibly know that skreigh is a verb meaning to "utter a harsh abrupt scream" without looking it up in a dictionary except someone who speaks this particular Scottish dialect.
A final tip: When using the Mozilla Firefox browser (instead of Internet Explorer or another) an "add-on" for automatic dictionary search can be--well--added on [as of the time of this writing], and, since "The Merry Men" is available on-line through the Gutenburg Project, you can highlight any of Gordon's or Rorie's words, right click, choose dictionary search and immediately see what The Free Dictionary has as a definition. It is most convenient.