How-To-Vote cards are an integral part of Australian elections.
They exist because of the preferential voting system which requires voters to number the candidates on the ballot paper in order of preference.
By issuing how-to-vote cards, candidates and political parties aim to maximise their vote and disadvantage their opponents.
The image below is of a how-to-vote card issued in the Western Australian state election on February 20, 2001. The card was issued by the ALP in the Legislative Assembly district of Vasse:

Note that indicating a preference for each candidate on the ballot paper is compulsory.
The ALP candidate, Ross Bromell, has given second preference to the Greens candidate and third preference to the Democrats. Pauline Hanson’s One Nation is placed last.
On the right hand side of the card, the ALP has shown preferences for the Legislative Council election in the South West Region. Council electorates are multi-member, so the parties lists of candidates for voters to choose from.
The image below is of a how-to-vote card issued by the ALP in the Queensland Legislative Assembly seat of Cook for the February 17, 2001 state election:

Note than Queensland uses the optional preferential system of voting where voters are not required to number each square.
In this election, the ALP simply advocated a “vote 1” policy. This is because the ALP will almost always come first or second in most electorates and its preferences will never be distributed. The ALP is more concerned about receiving the second preferences from other parties and candidates.
The image below is of a National Party how-to-vote card issued for the Surfers Paradise by-election in the Queensland Legislative Assembly on May 5, 2001:

Because there were 12 candidates in the by-election, preference allocation was particularly important. The seat was won by the independent candidate, Lex Bell.