My wife ( she is Thai she is 22 years old ) is going to be traveling with my daughter ( My daughter is 2 years old and holds British with British passport ) to Thailand in November, they will be staying about six weeks, she will need a visa. We live in UK. What do we need to do to get this visa. Can it be done by post. My wife has a residents visa here in the uk.

The Embassy in London is said to dislike mail applications, but your wife could contact them for advice.  Consulates all over the UK issue by mail.   You don't say where you are, but look at the Royal Thai Embassy London website for addresses. The visa's you require are simple 60 day tourist visa's...no drama