Add a command bar action.
Set the Display name to "Print" and give it a nice print icon.
Configure an action type.
Configure the "Open Web Page" action to open the Display Form of the current list (in this example, “Expenses” is the list) by passing the ID & action=print as an additional query string parameter.
Url: [[@Web.Url]]/Lists/Expenses/DispForm.aspx
Additional Query String: ID=[[ID]]&action=print
Open in: New window
Go to the Display Form customization and add an After-Form Load action of type "Execute Script" 
The URL parameter from the Open Web Page action was "action=print," so we added this condition to the Execute Script action.
The script itself is plain JavaScript. The first two lines add some CSS styles which will hide the command action bar on the printout. The following 2 lines print the content of the form, and the last line closes the additional tab that has been opened by the command bar action.
Title: Print form if action = print
Condition: [[@Page.GetQueryString('action')]]=="print"
Type: Execute Script
var style = "@media print { .ms-CommandBar { display:none;} }";
You should now have a Print icon at the top of your customized form. Upon clicking the icon, a new tab with a printing pane will open.
For forms with sublists or fields placed horizontally, a great alternative would be the Document Generator, which allows you to place each field where you want it and supports sublist items in a table.