The same top-to-bottom way you'd read a western comic, only moving from right-to-left instead of left-to-right. So, the first caption or word balloon will be closest to the top right of the furthest right panel and your reading order progresses down and left from there. At the end of each panel row you start the next from the furthest right panel progressing left.

Be aware though that it's only in the last decade or so that English translations of manga have preserved this right-to-left reading order - editions from the 80s and 90s used to be 'flipped', artwork and all, for left-to-right reading order. Some of these are still available, like the Dark Horse 'Akira' paperbacks, or even the contemporary Drawn & Quarterly editons of Yoshihiro Tatsumi's gekiga works ('Abandon the Old in Tokyo', etc.)

You can obviously tell which reading order the publisher's opted for by seeing where they've placed the front cover - if the spine's on the left (when the book is shut) the manga has been flipped to be read left-to-right, and if the spine's to the right it's printed in the proper reading order. But this can be confusing when reading a digital edition where's no spine to clue you in - especially in those manga that open with a string of silent panels.

MangaPan