Also the weapon you are going to dismantle needs to be the same type as the weapon you are upgrading. The weapon to be dismantled also has to be unlocked. A blue can be dismantled to upgrade a purple but the blue has to have a higher attack value. Don't bother infusing unless the difference in attack values is more than 10. You won't get much of a change.

You need to have a primary weapon with more damage than your Suros. Then just inspect your Suros and hit the infuse button. The other weapon can even be rare.

Materials needed for infusing: Legendary marks Weapon Parts Glimmer Motes of Light Plus, one weapon currently with higher attack then your Suros. Example: Suros at 280. Random Rare pulse rifle 299. Infuse the Suros with the random pulse rifle to make it about 295 attack using the correct materials. Done deal.