Just loook carefully there really no right and wrong or steps on doing it, here let me try to give you some step:-
1) indicate where positive and negative is, the negative side should have a white stripe or some sort this is the (-) side. and obviously the other side is (+) side.
2) Solder the (-) side on the negative terminal of your speedo
3) Solder the (+) side on the positive terminal of your speedo
4) Ensure you insulate any expose terminals with heatshirn or something just in case a piece of metal etc.. lands on the joints
By the way didnt the speedo come with instructions ??
I hope this helps. Anywayz enjoy and show them what TRF power can do heheheee