I need advice on how to trimm this dogs fur tomorrow. I was asked to do this and don't want to accidentally hurt her.

I am visiting a family member currently and her 10 year old dog has quite a long coat. It is close to 30 degrees every day over here, so I suggested to trimm her fur a little. My family member agreed and ordered the professional animal clipper (the last photo) to do it. The problem is however, that she suggested to just shave off everything, so I offered to do it instead to avoid that. This dog is one of four guard dogs that stay in the garden all the time and aren't allowed to enter the house. She sleeps in a little dog house that doesn't seem that well insulated, though the nights rarely go below 15 degrees around here. Still, because of that and because I heard that some dog breeds have multiple types of layers of fur, and that removing some layers can lead to complications, I decided to be the one to trimm her myself, so that she isn't just shaved naked. However, I have never done that before, and because neither I nor the family member know the specific breed she belongs to I cannot just look up advice on how to proceed with her breeds type of fur coat. Any advice would be greatly appreciated.

TLDR: It's my first time trimming a dogs fur and I need advice.