You've just identified the fundamental problem everyone has in trying to make a nice object: not enough information about the object to be modeled. With what you know know, it will be really crude. Do you have one of these chairs? It usually takes way more time to gather the data than it does to execute the model.
My solution is just approximate, but is close enough. Here are the steps once you have accurate data and dimensioned views:
The seat and back: make a complex profile of the unified seat and back section. Apply it using a wall that curves along the front of the seat lip. You get a solid plastic shape. See the Wiki for more on Complex Profile.
Make a Complex Profile to match the holes in the seat. Use it as a slightly tilted column to subtract the holes from the seat using a Solid Element Operation. See the Wiki for more on SEO.
Make a Complex Profile to match the holes in the back. Use it slightly tilted beam to subtract the holes from the back and to sculpt the top of the back using a Solid Element Operation.
Tapered legs: Here is where it gets squirrely, because Archicad doesn't want a tapered thing narrower at the bottom. Here you use a roof with a thickness the height of each leg. Use the roof edge angle control to precisely taper the legs. Or, you can use roofs to SEO slabs or something if the legs also tilt..... but we need to know the dimensions of the elements in order to be accurate.
Good luck. You will certainly learn a lot about Archicad.