While the parties - whether major or minor - want to make it easy for you to vote above-the-line, a below-the-line vote means that you determine how your vote is distributed after your first choice on the ballot, while an above-the-line vote means that your vote is distributed according to the preferences of your first choice party. (This may be not always be the same way you'd like your vote to be distributed.)
Anyone who passed 3rd grade maths is able to competently vote below the line. As long as you keep a careful count, your vote will be valid.
There are five easy steps:
(1) If there are any independents or minor parties that reflect your views better than either of the two major parties, vote for these candidates first, numbering each of these candidates in order, in whatever order you choose, beginning with the number 1.
(2) Then, after you've numbered your preferred independents and preferred minor party candidates (if there are any), continue to number the candidates from whichever major party you prefer (in comparison to the other one). Number them in whatever order you choose, which may be different from the way their party has numbered them.
(3) Now immediately after you've numbered the candidates for your preferred major party, continue to number the candidates from the other major party. (By doing this, your vote is highly unlikely to leak past your preferred major party.) Again, number these candidates in whatever order you choose.
(4) Then, number the remaining candidates for whom you haven't voted yet, in whatever order you choose.
(5) Finally, double-check your numbering. Remember that, to cast a valid below-the-line vote, you need to number each candidate in order from 1 to whatever the number of candidates on the ballot paper is, without either skipping or repeating any numbers. If you find you've made a mistake, go to the AEC staffer on duty and ask for a fresh ballot paper.
Voila! A valid below-the-line Senate vote.
A below-the-line vote is a good thing, regardless of the candidate or party for whom you choose to vote. A below-the-line vote means that you as a voter are using your intelligence as you cast your vote.