Here are some basic tips to create an informal style of letter writing -
Use vocabulary and expressions for informal letters. Bear in mind that informal writing sounds more like a telephone conversation with a friend. The speech is natural and relaxed. Example - How's things going lately?
Use contractions not the long form. Example - 'I'll' not 'I will'. 'She's' not 'She is'.
Try to avoid the passive voice as it creates a formal style.
Use beginnings and endings which are appropriate for informal letters. Examples - 'Dear Peter', 'Hi Peter', 'Dear mum', 'Best wishes', 'Kind regards', 'Lots of love'.
Use continuous tenses rather than simple tenses whenever possible grammatically as they create a softer tone.