How can an organisation conduct internal audits on an ISMS to comply with ISO 27001?

Your organisation should aim to conduct internal audits on the mandatory clauses of the Standard, Annex A controls, other relevant controls, and your own organisational processes that support the implementation of your ISMS.  The Standard promotes the use of a risk-based approach to auditing, i.e., the areas that are likely to suffer the greatest impact of a risk or is the most vulnerable should be prioritised.  You should aim to have completed an audit on every element of your organisation at least once over a 3 year period.