No need to list everything. Or put a value. No need to insure although you can if you want too although the PTT staff will probably look at you blankly. I have never insured a parcel and never had one go missing. You could put a list of the contents in the box for your own reference but it's unlikely anything will go missing.
Check the box that says other (diger) - I'll check with Dogan what you can write and let you know to make it clear it's used personal effects.
Choose and check the box that says surface (yüzeden) not airmail it's much cheaper.
You will need to do a form for each parcel, it is all quite straight forward - sender, recipient (person who will sign on receipt) contents as in personal effects, tick surface or air mail, sign and date. That's it.
The parcel will be issued with a tracking number and will arrive 10 to 15 days later, usually nearer ten days but can also be sooner and it will need to be signed for by the recipient. If they aren't in Parcel Force if that is what they are still called, will leave a card and it will have to be picked up from the nearest Post Office Depot or arrange to have it delivered at another time. Make sure it is collected in good time if this happens otherwise they will send it back to you and the return journey will take a lot longer.
In theory you are supposed to leave the box open so the staff can have a look inside and make sure it's not something it shouldn't be and then they will seal it for you. I don't do this and don't have any problems, they will ask what is inside though. Make sure boxes are strong and sturdy.
It is an excellent service and I can highly recommend it.