I am working on my instrument rating under part 61. I need 40 hours of simulated IMC and 20 of those hours can be in a certified AATD simulator like a redbird. Do I need a CFII to sign off on those hours in the redbird or can I log those hours myself and count them towards my 40 hours of sim IMC?
Yes, you need an instructor. 14 CFR 61.65(i) (emphasis mine):
Use of an aviation training device. A maximum of 10 hours of instrument time received in a basic aviation training device or a maximum of 20 hours of instrument time received in an advanced aviation training device may be credited for the instrument time requirements of this section if -
(1) The device is approved and authorized by the FAA;
(2) An authorized instructor provides the instrument time in the device; and
(3) The FAA approved the instrument training and instrument tasks performed in the device.
You may be thinking of maintaining IFR currency using a simulator. That doesn't need an instructor, per 61.57(c)(2).