The typical approach is like so:
Assume 1-5 for stats, and 1-5 for skills, as in WoD type games.
If we assume a 2 in stats, and that 2 in skill is pretty decent, thats a 4. 4D6 roll on average 14.
So we might go with the average, and say that a typical "trained" difficulty is 14. Depending on how hard it is to raise stats you might consider a truly expert problem to need 4-5 in both stat and skill, or you might say 4 skill, 3 stat and arrive at a difficulty based on those total pools, as guidelines.
Having an advantage could be represented by:
Add 1-4 to the total.
Add a D6 to the total pool.
Roll 1 more die than the total ignore the lowest 1 die.
Note you could also divide the total by 5 and get approximately the same success count as counting 5-6 as a success. I personally like adding up totals and dividing, which makes the stink of rolling one under a success less.
But really as long as it is done the same for all characters, its a "fair" system. After that, its a question of how does it feel, how easy is it to use, and how big are the jumps when adding a die, or a +1 result.
With adding the dice, you can also do more detailed things like say that every skill level is worth a flat +4, and only the related stat is rolled.
Also: Star Wars by West End Games, runs on a free system now called D6, has lots of examples of how they handle dice and difficulties and so on.