How do you summon a Yugioh fusion card with a really high level and for Ex blue eyes ulatmate dragon?
A Fusion monster's level is almost entirely irrelevant.
The Fusion monster will say what its components are, so for BEUD, it is 3x Blue-Eyes White Dragon. So that means if you have 3 Blue-Eyes White Dragon in any combination of hand or field, as well as a fusion card like Polymerization, then you can fusion summon the BEUD.
Fusion monsters may have special summon text is their summoning method is different from above, like the Gladiator Beast fusions or Elemental HERO Neos fusions that do not use Polymerization.