Step 1: Pieces You Need. Meter, Drill and Screw
Then search in you tool box for 2 pieces of plastic or metal around 1cm long (you can use screw nuts for example or cut 2 pieces of plastic from an old pen). This two pieces must have a small whole in the midle and be as more wide as posible to adjust into the whole of the curtain bar. It is just to prevent unwanted movement and noise when moving the curtain. If you dont care about you can even use the screw directly to the wall without the plastic piece, just leaving the screw 1cm out of the wall.
Step 2: Adjust the Curtain Bar
When you have both sides screwed with the 1cm plastic pieces to the walls (and an extra hook from the ceiling at midle distance from the walls when the bar is longer than 150 cm or curtains are too heavy), measure and cut the bar (try if it comes in), then introduce the curtains, hold the bar and curtains.
A last step is required...
If too stiff the bar and you had to cut this lesser 1cm, then when you introduce the bar in both sides, you need to put around the plastic pieces a rubber ring 0.5cm thick (less is also usefull but curtain bar will move a litle because the gap). If you do not so, curtain bar will easily fall down..
Dont permanent fix the rubber rings to allow bar removal to wash the curtains once in a while (they get very air polution dirty and is healthy to wash them as often as the polution of your city is, in my city I have to wash them once bimonthly).