designing sampling plots on QGIS

Dear Seokwang, in order to create a sytematic or random grid using QGis i would suggest you to have the shape file of the area/region/country which you need the grid. Save that shape using the projection that better keep the area of the shape (for example I usually save my shp using Albers equal area). Once you calculate the number of plots you want for your grid you go on Vector -Research tools - Regular point ( if you want a systematic grid), in input extent you can select Use layer/canvas extent, in point spacing/count you have to calculate in decimal degree the distance between each points (https://en.wikipedia.org/wiki/Decimal_degrees) and Run it. The output will be a grid generated on the layer extensions, you can use the Clip tool in order to cut the grid on the shape of your layer. Last time i tried to generate a grid I used QGis 2.18 Las Palmas and it did not work. Try to update QGis to the last version if it doesn't work with las Palmas and let me know. In case we can help you to generate a grid using GEE or ArcMap. Thank you, Chiara from OpenForis team

The preferred way to generate grids is by using this Google Earth Engine tool : Collect Earth Grid Generator