Rich'sToys is essentially correct although his method is wrong. There should be a plug on the cylinder near the top (rod end) where fluid can be added. By leaving the plug out as Rich has suggested you'll blow air and oil out all over the place. The idea of cycling it several time is correct to remove the air but the plug needs to be one which allows for the escape of air or something as simple as a pipe plug with a small hole drilled through it and a cotter pin placed in the hole. This allows the air out but little oil and keeps some of the dirt out. Not the best situation but better than a big belch of oily air passing.
And be sure that all ther fittings are tight and not leaking first or the air will come right back. This applied to a closed system only. If your hydraulic system has a reservoir then allow for a 20% expansion and air displacement so the solid plug is OK in lieu of a breather cap.
