On a Mac...
- under Page Setup, Select your paper size and Orientation
In the Print panel on the right...
- Layout Style - choose Single Image
- Layout - select the Margins for the print; they will determine a maximum Cell Size, which you can alter down from using the Cell Size adjustments;
- The image you see may not fill the dimensions of the cell determined by the margins. It is the image as cropped in the Develop module. If you wish to fill the cell (allowing the Print module to crop from the cropping you may have done in Develop), then go to Image Settings (above Layout) and select "Zoom to Fill"
Under Guides, I find it helpful to "Show Guides" and often have them all turned on, but that be confusing until you see what each one shows you.
As stated above, LR does the arithmetic for you with regards to pixels. Under Print Job, enter the print resolution best for your printer - Epson, 360ppi; all others (I believe) set to 300ppi.
Also, be sure to download the printer-paper profile (unless, of course, you create your own) so it can be selected under Colour Management.
Hope this helps get you started! I highly recommend Scott Kelby's book for Lightroom 5. It gives step-by-step instructions, with photos and screen captures at each step to help you. It sure beats trying to find just the right clip in the video series!