How do you make a loved one with dementia understand the importance of daily hygiene?

As stated by others, you can't convince her. You have to supervise daily dressing from now on, and as others said, the shower aide is vital. Those people can do miracles you can't match no matter how hard you try. I was bathing my mom myself, and she still smelled bad. One session with a shower aide, and she was fresh and truly clean for the first time in years.

It isn't going to get better, only worse, so stop banging your head against a wall about reasoning with her.

I'm sorry, I think I phrased my question incorrectly. Of course I've realized that a person with dementia will not understand and I appreciate all the advice I have received this far. You are all correct - it will only get harder.

Unfortunately you will not be able to do that. she has lost that ability to understand and reason why hygiene is important. As the disease progresses she will lose the ability to do other things as well. Get yourself some help and do as people here have suggested ahead of time. Get a shower aide to help your mom. Your mom will not be happy at first and she will most likely refuse to argue and fight and cry and whatever else. But the aid knows how to handle it. She’s not always no matter how much she understands let her know that you’re there for her and that you love her.

You can't "make" someone with dementia understand the importance of.................(pick your problem and insert here)
She has dementia it is up to the caregivers to do what needs to be done, convince her of what needs to be done.
It could be take a shower, change clothes.
In this case replace all her underwear with the incontinence product of your choice.
You can hire a Shower Aide that will come in 2 or 3 times a week and shower her, change her, brush her teeth.
You or your husband have to do the monitoring of ADL's on a daily basis. Brush teeth, clean her after the bathroom, make sure she has clean clothes. Easiest way to do that when she goes to bed remove her soiled clothes and replace them with clean ones.
This will not get easier and as she declines more and more tasks will be added to what she can not do.

Sorry, you can't. She has probably lost the ability to reason. Probably processes what you are saying very slowly. To me they get like 4 yr olds. Wanting everything their way.

I never asked when I dealt with my Mom. Most of the time No was the answer. So I told her, time for your shower or just walked her there. I dressed her so her Depends got changed every morning if she hadn't had an accident prior.