There are complicated ways of doing this, but the naïve way is as follows:

First you need a corpus of text that's grammatically correct

Each node in the chain is a word or piece of punctuation. Each word has a certain probability of being followed by every other word in the corpus, including itself. There are a few different ways to start the sentence. One approach is to start from the node for the punctuation mark ".", and only selecting a following node that is not a period, since sentences don't tend to start with punctuation. From there, use a random number generator to pick a following node based on your probability matrix, rinse, repeat.

If you'll notice, there's no guarantee that it will be grammatically correct. There's just some statistical likelihood that it will be.