Step 1: Adding the position to the correct xml fileGo to your template's root folder (usually <Joomla-Root>/templates/<your-template>) and open templateDetails.xml in a text editor. Scroll down 'til you find <position>-tags. At the end of the row of position tags, add another one (just clone the last tag and increase the number by 1). Save.
Step 2: Check if what you just did workedGo to your Joomla! backend. Open Extensions > Module Manager. Select a module (open the module item by clicking on it). Click "Select Position". Does the position you just created appear in the table? Good!
Step 3: Name your position (optional)Now you can give your position a name. The name is what appears in brackets in the position table. Go to the <Joomla-Root>/administrator/language/<your-language> folder. Open file <your-language>.com_modules.ini (for example en-GB.com_modules.ini) in a text editor. Look for a line that reads 'COM_MODULES_POSITION_POSITION-18="Position 18"' (where 18 would be the number of your new position -1) and add a line for your new position (i. e. 'COM_MODULES_POSITION_POSITION-19="Position 19"'). Save. (By the way: you can specify whatever you want to be the name of your position. It doesn't have to be "Position -<NumberOfPosition>")
Step 4: Insert the position in index.phpAll that's missing is the tag that specifies where your position should be included. Go to your template's root folder (usually <Joomla-Root>/templates/<your template>) and open index.php in a text editor. Go to the position where you want to include the position and insert a tag similar to this one:
<jdoc:include type="modules" name="position-19" />
Replace "position-19" with the correct information you entered in the xml file (this is NOT the name you specified in the language file!).
Step 5: Assigning a module to the positionWell, you should know how this works. Just open Extensions > Module Manager in your Joomla! backend, open a module, click "select position" and select your new very special own position. Reload your page to see if everything worked (clearing your cache might be necessary).
Let me know if you had any problems with this!