Marking is always done in the sitting position. The lid crease is marked first from punctum to lateral canthus. If a crease is present use that, otherwise if there are multiple creases use the one which is complete and at the correct height. Generally lid crease is at 8-10mm from lid margin in Caucasians and 5-7 mm from the margin in Asians. Make sure the crease is symmetrical on both sides. Medially, you should not extend medial to punctum and laterally stay within the line joining the lateral canthus with lateral end of the brow. If you have to excise more medially then go up at an angle of 15 degree .Then the amount of skin to be excised is marked using the skin pinch technique. The skin is pinched with a non toothed forceps and the amount of skin to be excised is gauged.
If there is a lot of lateral hooding I mark the lowest point where the eyelid fold falls .This is a trade off for having the scar visible on the thicker skin laterally which is explained to the patient pre operatively.
Make sure there is atleast 20 mm skin between the lid margin and the lower end of the brow (be cautious in threaded brows) to enable eye closure. You can pinch the area to be excised and make sure the eye closes well after pinching.
Generally the scar heals well in thin skinned Caucasian population. However it may be more visible in Asian eyelids.