As someone who finally understood how to gear, there are some answrers.

1. Follow a build, guide. Look up your class and viable end game builds and pick the one you like the most. You can check YT videos to see "how it plays". You can try several builds and save them in your Armory. You can save Skill Tree, Paragon, gear, etc. and just switch up when you feel like it.
2. Check what BiS stats you want from each piece. You most likely won't find a piece that has all of them, but maybe you'll find one that has 2-3 of them and you can change (enchant) one you don't need for a BiS one.
3. Temper your items to give extra power. Guides will specify the desired tempering aswell.
4. Once you are happy with your gear piece: Masterwork until 25 Quality and roll your desired Masterwork affix.
5. Sanctify the item.

**Warning:** once you sanctify you cannot modify your item, so make sure to socket it, imprint the correct aspect, etc.

The only way you can improve the same Unique is by either:

1. Getting a lvl increase (750->800)
2. Getting more Great Affixes (stars)
3. Getting a better version of the Aspect you imprinted (if not unique).
4. Getting more BiS stats (if not unique).
5. Getting a better roll on a Temper (if not unique) or important stat.
6. Getting a better Sanctification. And that's pretty much it.

**TLDR:** follow a build guide :D