You can get badges from the Badge vendor found in most every town, and can earn them through the Trials. In fact, it's pretty self explanatory.
There is really no real "guide" to this, but I will give a tip. Avoid buying badges until you've unlocked all of the banks and getting the achievement for storing 10k gold. Badges aren't necessary outside of the first few you can buy or get from trials, but they are pretty fun and/or OP.
I only add this tip in case you don't want to grind for AGES to get 10k gold after being broke from all of the badges you've bought. B-Line to each village and store that money!
If you're capped out at the bank, do whatever you want with the remainder, where that be saving it for the next town bank, weapon upgrades, buying badges or badge space.
Either way you have to grind, but just keep that bank achievement in mind if you wanna grind a little less.