Decide what style of samba you want your tune to use. Styles include partido alto, pagode, samba reggae and the bossa nova. Each style of samba has its own unique tropes and rhythms, and knowing what type of samba you are creating is vital to the authenticity of your tune.
Decide what instruments you are composing the samba tune for and assign each of them a staff line on your computer or staff paper. Commonly used instruments include drumset, hand percussion, bass, piano, guitar, horns and vocals.
Choose a time signature for your samba and write it on the left side of each staff. Most sambas are in 4/4 or 2/4 time, but some styles also use 3/4, 6/8 and other meters.
Choose a key signature for your samba tune, including whether you want it in major or minor mode, and write the key signature on the left side of the staff for each melodic instrument.
Write the theme of your samba tune as you hear it in your mind on whichever instrument or instruments is most appropriate. For example, if you are hearing a specific rhythm or groove, write it on the drumset and hand percussion staffs. If you hear a trumpet melody, write it on the trumpet staff. Every composer has a different order in which they compose, but it usually starts with one specific theme, and this is what you should write first.
Develop your theme by adding harmonies and rhythms in the other instruments. Like jazz music, many samba tunes follow a certain format. For example, your tune may have an "A" section that features the melody, a "B" section with a counter melody and a solo section for improvisation. A typical format to start with is "A-A-B-A-A(solos)-A-(solos)-B-A." In this case, the harmony and rhythm in the middle "A" sections is the same, but different instruments take turns improvising rather than playing the melody.
Add an introduction and "tag" (closing statement) to your samba tune. The introduction can be an elaborate unison phrase with all of the melodic instruments, four measures of the drumset playing the basic rhythm or anything else you can create. The tag is typically a repeat of the last four measures, but is also subject to your creativity.