If an effect instructs you to "double" a creature's power, that creature gets +X/+0, where X is its power as that effect begins to apply.

If a creature's power is less than 0 when it's doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, suppose you control Infesting Radroach, a 2/2 creature. If an effect has given Infesting Radroach -4/-0 so that it's a -2/2 creature, doubling its power gives it -2/-0, and it becomes a -4/2 creature.

If you "double" a creature's power more than once in a turn, each doubling effect applies independently. For example, suppose Junk Jet is attached to Infesting Radroach, a 2/2 creature. If you activate Junk Jet's second ability, Infesting Radroach will get +2/+0 when that ability resolves, making it a 4/2 creature. Doubling its power again will give it +4/+0, making it an 8/2 creature.

You can't sacrifice a Junk token to pay multiple costs. For example, you can't sacrifice a Junk token to activate its own ability and also to activate the second ability of Junk Jet.

You pay all costs and follow all normal timing rules for the card played from exile with a Junk token's ability. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.

Some spells and abilities that create Junk tokens may require targets. If each target chosen is illegal as that spell or ability tries to resolve, it won't resolve and none of its effects happen. You won't create any Junk tokens.