Practice with mapping in general, start a map from an idea, then work off of it until you think you've made something worthwhile. It took me countless unreleased maps, detailing practice, and all-nighters to get where I'm at(and there's still a lot to improve on) I would say that finding a good sense of scale is the most important when starting out. Tips for getting scaling right is to look at decompiled official maps and seeing how big they are. You can take doorways from official maps, and put them in your own levels and make rooms around them. Or just have models placed around your level(I.E playermodels) when constructing your level.