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first step was International maritime signal flags:

THE FIRST 
STEP IS 
DONE NOW 
DECRYPT 
JTPMWFEUIF 
GJSTUSJEEMF

the second step was caesar cipher (Rot 1):

ISOLVEDTHE
FIRSTRIDDLE

Caesar ( Rot1)

ABCDEFGHIJKLMNUPQRSTUVWXYZ
BCDEFGHIJKLMNUPQRSTUVWXYZA

congrats!
you solved it complete. :)

Caesar?! Really??? Shame on me! I just suggest I was somekind of drunk even if I dont really drink xD

Tried it but somehow I must forget 1-2 steps xD

If you writed this way:

ISOLVEDTHE  FIRSTRIDDLE
JTPMWFEUIF  GJSTUSJEEMF

you can see how easy it was ;)

yeah, just sprinkle some more salt into my wound :P