I'm not quite sure, as it is written in oldern english, but I think it means that writing is a type of art that also takes practice and it is not by chance that writing is mastered and written at ease (and then he compares it to people who have learned to dance, dance ease as compared to others who have not). Then this is the part that i'm not quite sure about - but I think he's saying that harshness is not supposed to give offence, it's supposed to be instructions and it must be taken to heart.
These lines come from Pope's poem "An Essay on Criticism", and is part of his discussion of what constitutes good writing. The point of the last two lines might be paraphrased like this: just because a poem is not awkward or harsh doesn't mean it's actually good or worthwhile: there needs to be a close relationship between WHAT is being said and HOW it is being said. Pope was very strict, demanding, in his literary judgments and standards.
It is a juxtaposition of words, just as Art is a juxtaposition of strokes. But words are the product of the human brain. Even this is unclear. Parrots (and other birds) speak (presumably having no idea what they are "saying").
A good writer is so because he has learned and practiced his trade. There is no luck involved in writing. Like opinions, I suppose, those sitting on the fence would not offend anyone with their opinion except bore everyone with it. It doesn't invite a discussion, an exchange of differing ideas. So too it is with writing, just because it is not controversial doesn't mean the writing is good. When people write, generally because they have an idea or an opinion to share - we write to stimulate the mind into a discussion. That should be the goal, I think.
Art is mastered technique. When you have practiced enough that the underlying technique becomes a second nature. And, modern artists would say that the mastering of the technique gives you the right to go beyond it.
This is the poetic way of saying good artists create and great artists steal. Are my words understood by any one else except me, or is this just soliloquy?
It say that writing is a skill that takes practice, just like dancing. The "'Tis not enough.." means Alexander is saying "Well, I don't mean to criticize you guys. and the lats part is him saying, "but I'm not the first talk about this."