"用斜体字排字" can be translated as "set (the text) in italics" or "print (the text) in italic type" in English.
Here are some example sentences to illustrate the usage:
Please set the title in italics.
(请将标题用斜体字排字。)
The important terms in the text are printed in italic type.
(文本中的重要术语用斜体字排印。)