Use a dedicated crimping tool. It has jawed recesses with raised ridges that press a pattern into the sleeve for a firmer hold.

If nothing is stated on the packaging of your leader material or the sleeves themselves, a good rule of thumb is to go for an inner diameter twice that of your leader material.

When you want to build your own leaders or stingers from wire, titanium, heavy fluorocarbon or hard mono, and tying knots is not an option, crimp sleeves are used to secure the connections.

No, once crimped they hold fast and cannot be undone.

That depends on the tool. If it has the right jaws in the correct size, then yes.