Standing up is the normal way most people measure. You can also measure laying or sitting down.
For length, you want to measure using the bone pressed erect length (BPEL) way of measuring because your fat pad can cover your length and is the most accurate way to measure. You also measure from the top of the penis, from the picture thread you see that’s how other people do it and that’s how you should do it for an accurate way to measure length.
For girth, you want to measure tight around the mid shaft (MS-EG) of the penis because that’s what most of your penis’s girth is the same. There is also base erect girth (BS-EG), there is also tip of shaft* erect girth (TS-EG), and there is also head girth which not shaft of the penis.