First you will have to wait until the Soldier has been in the unit for 90 days. See AR 600-8-19 paragraph 10-6:
A Soldier must have served in the same unit for at least 90 days prior to being reduced one grade for inefficiency.
After that, you need to make sure you have all your documentation together. If you haven’t been counseling often, start doing it now. You need to document what corrective training has been conducted as well as the outcome. Be sure to fill in the assessment blocks on these statements. You must show documented efforts to help the Soldier improve.