How do I add a drain plug

Well I'm getting a trailer and I now need to add a drain plug anny tips? Thanks

You can have an aluminum drain tube welded in or you can buy a plastic fitting and plug that is mounted with screws and sealed with 3M5200.

Are the plastic ones reliable

If you don't already have a drain now, you could add a BRASS drain
with no welding or epoxy fumbling around.
This is what I put on my wife's old Sears 1236. The original drain had
the standard pipe with rubber plug that had rotted out. I repaired that
hole and installed this brass drain in the floor.
Okay, the flange sticks out about 1/2" at the bottom when installed.
If this is an issue, as it was for me, here is what I did ................
Install the drain flange withOUT any gasket or caulking. Screw it in tight.
Turn the boat over, take your 4" grinder with a cut-off blade and shave the
protruding flange AND plug down to where you are happy with it. Let it cool.
Remove the assembly and clean up any rough grinding.
Apply your caulking and re-install . . . . .
You can get this brass fixture at most boat stores, e-bay, amazon, etc.
I paid $8 for mine at Bass Pro.