To unlock the Buckle Up trophy in Red Dead Online, you need to earn the gold award in 5 different challenges in the Awards menu, which you can find within the Progress menu.
This is probably not going to be as difficult as it sounds. A lot of the gold buckles are a big grind as you need to repeat actions multiple times, in some cases 1000 times. Others require a lot of skill. However there are some that should be pretty straightforward.
First off, there's the Free Roam Award which only requires you to complete 10 Free Roam missions to get the buckle. You need to do that anyway to get:
Here's some other simple ones:
FREE ROAM: Complete 4 Free Roam missions in a day -- head out away from other characters and pick up missions to have the best chance at this. Obviously try not to die.
GENERAL: Reach Maximum Honor -- you can actually hit this really quickly, much more quickly than in the base game. Looking after your horse nets you good Honor, and the first few story missions after the tutorial (i.e. after you've set up camp) are all honor-based. If you are on a team that are like-minded and vote for the honorable options in each case you'll likely get this buckle shortly afterwards.
TRADE: Earn 100 Gold Nuggets -- this is Rockstar's premium currency and as you might expect, earning it without paying real money is an awful grind. Most times you find or are awarded gold nuggets we're talking 3 or 4 at most. However, I received a Treasure Map early on (shortly after wrapping up the honor missions mentioned above). It's unclear if this is true every time, but the treasure I discovered was 1 Gold Bar, which is equivalent to 100 gold nuggets and granted me this reward.
HORSE: Achieve maximum bond level with your horse -- this works in the same way as the base game, so again just try and stay away from malicious players and this should be achieveable.
HORSE: Purchase 5 unique horse breeds -- you can work towards this while pursuing the below trophy:
TRADE: Sell an item to three shop types -- You'll need to sell to a Butcher for a trophy anyway. It should be pretty easy to sell items to a Fence and a Trapper as well.
SURVIVALIST: Taste 20 herbs -- Cons: you need to taste 20 unique herbs. Pros: "Herbs" is misleading, berries and flowers seem to count too.
SHARPSHOOTER: Acquire 19 guns -- I had 4 by the end of the tutorial so this is unlikely to take too long, especially if you're heading for Rank 50.
If you're a good shot, there are some Hunting and Sharpshooting awards that are less of a grind:
Kill 3 enemies with a sniper rifle headshot in 10 seconds
Kill 1 flying bird with an arrow
Kill 1 player with a bow from 100m
Kill 10 enemies in a single Dead Eye use (Note -- that's enemies, not players. You can do this with NPC enemies)
Kill 50 enemies consecutively with the same weapon (it appears you have to do this in a single session, my counter reset when the game crashed)
Kill 6 enemies with a revolver without reloading
Get 5 gold medals and the trophy should unlock.