It's actually very simple (Rebecca you do it all the time, you just maybe forgot that what it's called)
Creating an internal link is identical to an external link - the only difference is where you're linking to - your own site (internal) or some other site (external).
First write the words/sentence you wank linked (e.g. "read a chapter") then highlight that text with your cursor and click on the "link" symbol (it looks like a small chain-link) in the toolbar above the write box (OR if you use the HTML tab, the button that says "link")
In the dialog box that opens, enter the URL that you want to link TO, in this case, a page that you've created that has the sample chapter on it (it may be something like http://www.yourdomainhere.com/my-sample-chapter/)
If you're in the Visual tab dialog, you can enter the "Target", which is to say, do you want the link to open in the same window or a new browser window? If it's an internal link you should always open it in the same window. For an external link, my personal opinion is that you should open in a new window, but there is a lot of controversy over this method, so you may want to do some google searching and read the reasons pro/con to decide which you like better.
If you're creating links in the HTML tab, you have to enter the Target manually, so for people unfamiliar with HTML I recommend using the Visual Tab to create your links.
