One of the most common mistakes that parents make is to teach children capital letters before they teach the lower case. It is so common that when children enter into my prekindy classes they are already writing their names in capitals and recognise all the captial letters. This is so frustrating for me as I have to teach them only to write their first letter in capital and the rest lower case. It is like unlearning a bad habit for the children - not impossible but a long process.
You might wonder why it is a bad thing to know capital letters first. The answer is simple: WE DON'T WRITE IN CAPITALS (unless we are shouting, and I'm not shouting, just demonstrating a point). For many children who are taught capitals first, some of the letters will be hard to change so they end up writing like this: I weNT To The shop ANd I boughT A Toy. It is far better for children to learn the lower case first so that when they write, all their words will contain lower cases. After they achieve this they then learn that capital letters are for the beginning letters of proper nouns and also at the beginning of sentences. At which time it is easier for them to switch over.
Having said all this, I do teach children capitals but only after they have fully learnt all the lower cases which is usually in term 4 (after 3 terms of lower cases). When I teach I emphasise that they are only to be used at the beginning of names. Children will have caught on to this already as they have learnt to use a capital for their names and also recognise their classmate's names. (The teaching of capital at the beginning of sentences occurs in kindergarten so it is not until the next term.)
Learning capital letters is easy because some the letters are the same (c, o, p, s, z). The only letters that are completely different are a, e, r, while the rest are similar. Just have a chart with both capitals and lower cases on it so that children can compare. I don't teach the letters that are the same (I just point out that they are the same). The similar letters are easy to teach as children will recognise them. Recognising capitals letters is easy but writing them is harder so have the children write them as well.
Below are the crafts for the capitals that I teach (although I do teach A but have forgotten to take the photo).