Your older dog may balk and be resistant to having a toothbrush put in his mouth at first. If you insist and show pressure, all will be lost. Your good intentions for removing plaque will take even longer if you start off with a brush. First of all, sit close by preferably at eye level and let him smell and taste the toothpaste. It will be unusual for him to ignore this, but if so put a little peanut butter on your finger instead. With your other hand, using a couple of fingers gently lift and pull back his upper lip.
Quickly run your finger all the way along the gum line and let your dog lick off the toothpaste or peanut butter. Praise him and repeat with the lower lip using just the toothpaste. Stop, praise and relax for a while before doing the same thing on the other side of his mouth.
This may take a few days' practice, sometimes more, sometimes less. You'll be able to tell when he's ready for the toothbrush.
If you still feel your dog may be resistant to a toothbrush try using a finger-brush. This will be far less 'invasive' and since he's used to your finger by now it may be better for you both. Put a little toothpaste on the brush and let him smell and lick it first before you start brushing.
Then, with your other hand gently lift and pull back his upper lip. In a circular motion brush a couple of teeth and get up as far as you can into the gum line. Repeat this on the lower lip and the other side. Stop often, praise and remember to be gentle, playful and persistent.
Do not spend any more time cleaning your dog's teeth than you would your own.
Gels and sprays are also available, which continue breaking down tartar build-up overnight. For these to work best remove your dog's water-dish before cleaning and replace first thing in the morning.