I thought I knew everything there is to know in building awesome Gundam plastic models until I saw this interview of Asrul Hazimin and his collection of customized Gundams.
Watching his YouTube channel, you can see his meticulous process on how he goes about customizing and essentially making better each GUNPLA kit he is working on.
Gunpla Basic Panel Line Scribing
Gunpla Weathering: Rusting Effect
Gunpla Weathering: Rusting Effect pt.2
He only has a few videos on his channel, but his tutorials on GUNPLA is enough to get you started in building your own customized Gundam models.
Just take a look at some of the other Gundam models he worked on below.