You would use either Hiragana and Katakana. To spell it out.
Hiragana is used to write exlusivly Japanese only words. Katakana is used for writing foriegn words or in advertising.
I recommend using Katakana for your drawing as it looks much cooler (hence why its used in advertising) and is easier to draw/write.
Basically double letter sounds make up one character (except for vowls and a couple others).
There is 52ish of them but really there is upto 100+ depending how you look at it. Both Hiragana and Katakana use the exact same sounds but just write the sound differently (a few exceptions of course).
Anyways just do a simple search for a basic table. But the ones you are looking for are
Sa
mu
ra
i
a
So you would need six Characters.
Alternatly you could just use the Kanji for Samurai and add the double vowl A to the end using kata or hira.
Pretty easy really. If you write in Katakana to extend the vowl sound you use kinda like a hyphen after the character for a. As a long vowl sound for the A in Japanese would sound exactly like ahh.
I don't have japanese characters installed on work PC otherwise Id post it up for you.
This table is pretty good.
http://www.sf.airnet.ne.jp/~ts/japanese/table_k.html
And heres one for Hiragana
http://www.sf.airnet.ne.jp/~ts/japanese/table.html
Note double vowls for Hiragana are just writen twice you don't use the dash.