What is the ruling on advising others to perform Tahajjud and Qiyam al-Layl while I myself fall short in doing so, and do I bear a sin for giving this advice?

Night vigil prayer (Qiyam al-Layl) is supererogatory, not obligatory. Therefore, whoever abandons it without an excuse does not commit a sin. And whoever commands others to perform it but does not perform it himself, does not fall under the threat mentioned in the Hadith: "A man will be brought on the Day of Resurrection and cast into the Hellfire..." This Hadith and similar ones are specific to enjoining obligations and forbidding prohibitions. Whoever enjoins supererogatory acts and calls to them will be rewarded, and he does not sin by abandoning them himself. It is befitting for one who enjoins Qiyam al-Layl while not consistently performing it himself, to seek the reward for giving advice, and to intend to help his Muslim brother, for Allah assists His servant as long as the servant assists his brother. It is not permissible for a person to abandon enjoining good and forbidding evil under the pretext that he is negligent, for enjoining good is an obligation and is not waived by the doer's abandonment of it. The negligent person should strive to reform himself while continuing to advise his brothers.