This is being written in response to another request for help from a reader of my blogs.
The very first thing you need to do is to make sure that individual post page (permalink) is enabled. Sign into Dashboard, select the relevant blog, then click the tabs SETTING > ARCHIVING, and make sure that the drop down menu for "Enable post pages?" is set to YES. When this is done, each post of your blog will have an individual post and a unique URL. See screenshot below (click to enlarge):
then click "Save setting". For those in Blogger Beta, that's it, but for those still on Blogger classic template, you will have to republish for the setting to take effect. Of course, if yours had already been set to YES, the default, you also may not need to republish.
The next thing you need to do is get the URL of the individual post page of the post you want to link to. This can be done in various ways. One is to click the POSTING > EDIT POSTS tabs and a list of all your posts will be displayed.
Right-click on VIEW on the particular post you want to link to and select "Copy link location" (this is for FireFox. Wording will be different for different browsers, but you should be able to tell which one). The URL of the individual post page of that post will then be copied to your clipboard. If you want to make sure, you can always open Notepad and paste it there. (Note: I like to use FireFox for things like this. If I am in the process of preparing a post, and I don't want to lose that page, what I do is I right-click on the EDIT POST tab and select OPEN IN NEW TAB and I can get the URL of the relevant post from that new tab, then get back to the post editor tab).
If you are trying to put a clickable link on your post, while you are preparing your post in the post editor, type in the text that you will like to be displayed as a link, highlight the text, (see screenshot below. Click to enlarge):
then click the "link" icon and a pop-up will appear (See screenshot below. Click to enlarge):
Paste your target site's URL into the field or type it in, then click OK. If you have done what had been instructed above, the URL had already been saved in the clipboard, and all you have to do is to right-click on the box and select "paste". (make sure you don't have two http:// in the URL). When you are ready, click OK, and the HTML will be added to your post.
If you like to write the hyperlink HTML yourself, here is another way to do it: Type the HTML <a href="URL of your target site">Description of your target site, eg., the title of the post</a>
If anything is not clear, ask in the comments section and I'll see what I can do to help.