I would make sure if you are going to use a toothbrush, that the bristles are very soft. I would probably go with a good MF applicator, or if you don't have one, a good clean rag. Remember that Leather does not like to have abrasive stuff rubbed against it.
Try the MF or rag first. If that is not getting it into the cracks, use your hands and rub it in. Gentile is key. If you have a particular area that needs attention, just make sure to use a bit more elbow grease.
Best regards.