First, find the hole that is factory plugged with solder.
Unsolder it.
Drain the fuel out, and I like to let them sit overnight at least to "vent".
Re-solder the hole.
Get some water to a boil, then take it off the burner so it quits bubbling.
Stick a piece of vacuum hose of appropriate OD into the centre hole, insert float into water, hold down far enough so you can see the bubbles.
Hot water makes the air inside hot, makes it want to expand and bubble out.
Pull it out, mark the area with something like a felt marker or scratch it with an X using an xacto blade.
Set aside, do the next one.
When all done, clean and solder, and since it isn't wiring, acid core works (like for plumbing).
When all done, re-test until you don't get any more bubbles.
If you see no bubbles, remove from water, remove hose, turn float over, re-insert hose into THAT side and re-test.
I've fixed Lord knows how many carb and fuel tank floats that way over the years.