As Hugo says, the amount of effort required to get at the aperture blades varies hugely from lens to lens, with more modern lenses (i.e. autofocus) being typically somewhat more difficult than (for example) most large format lenses, which often don't even require tools.

However, to answer your question directly you can often (depending on the lens) do a quick and dodgy clean of oily aperture blades with a clean lens tissue and isopropyl alcohol (avoid q-tips; too many fibres end up on the aperture blades). To do this, fold the lens tissue into a small rectangle, tear one edge to act as a 'mop'. Put a few drops of alcohol on the torn edge, then very carefully drag it in a circular motion around the surface of the closed aperture blades. Use a similar process with a dry tissue to soak up the oily alcohol off the wet blades before it dries. The result will be slightly cleaner, albeit smeary aperture blades.

Note: This is a risky process and you can easily get oil on other surfaces in the lens, as well as dislodging aperture blades, which then require a lot more effort to reposition.

A proper clean would be done by removing the blades, washing/drying them and reassembling.

Also; the type of aperture affects the difficulty, with simple 2-blade square apertures being simple and 20+ blade complicated apertures like this one from an Apo-Ronar being quite tricky to get back together again:

And; oily aperture blades don't often actually cause any issues unless:

1. There is sufficient oil to slow the action of the aperture.
2. There is sufficient oil to migrate/evaporate onto other parts of the lens, such as the glass elements.