What advice can I get for applying for a Retirement Visa for my uncle?

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Your uncle has to get his required Thai bank account in person. You can help him in that respect by finding a bank that will open one for him, based on his visa to stay in Thailand. If it is a tourist visa, that can make it difficult, but not impossible. Some immigration offices require a Thai police clearance so he has to go to the police headquarters in person to be fingerprinted. You can take him to do this, if the immigration office wants it. He will also need to go to his Thai bank to get two letters from them. One showing at least bt800,000 in his account at least two months before he applies, and a letter stating it is his account, in his name only. He must do this himself, as he will be required to show his passport. At immigration, if you plan on getting there in the queue before they open, so you get done before noon, you can stand in the queue for him, and he can arrive a little before his queue number comes up, but if he is more than 30 minutes late, he'll have to start over in the queue. Then you could be there all day. Those are about the only ways you can help him.

The necessary documents etc. are listet on Thai Immigration or Embassy sites.

But your uncle has to apply in person.

Some immigration offices have docs in Thai and English listing required docs. May even be able to find them online.

Without any information about your Uncle it’s pretty hard to advise. Is he in Thailand? Does he have financial means to support the application? (Assuming you’re trying for an in country application) you can do most of the things needed for him, he needs to be there at the time of application, the bank account and stuff needs to be in his name. The visa is his and everything applies to him, not the person applying.