Once you are happy with the scale and position, click Project > New Print Composer.
A window will open asking for an ID/Title, however it is not required. Click OK.
A new window will open showing a blank page. To insert the map from QGIS click on ‘Add new map’ (see below).
Left click and then drag a rectangle to select the dimension of the added map. Once added the position of the map within the frame can be adjusted by selecting ‘Move item content’ (see above).
Click on ‘Item Properties’ on the task bar on the right. The specific scale can be selected here, and if you scroll down there are several other options including ‘Position and Size’. Here the dimensions of the map on the page can be adjusted in millimetres. Careful, as when altering the position and size the scale will also change.
Once you are happy with the image you can export the map as an image or a PDF by selecting ‘Export as image’ or ‘Export as PDF’ respectively (see below). And save within an appropriate folder.
Your final JPEG image should looks similar to this:
