STEP 1 : Parade Rest -  Practice several times until you can do it. Pretend / practice while you are holding your instrument.

RIGHT FOOT out and RIGHT ARM out at the same time and yell G!
RIGHT HAND goes to center of your back and yell H!
CHIN goes down and towards your RIGHT shoulder and yell V!
Notice how they hold their instruments. 
Flute/clarinets hold instruments the same way
Trumpet/mellophone/trombone/baritone hold the same way
Tenor/alto sax hold the same way. Bari sax just move arms.  not horn
Drummers move arms not drums

STEP 2: Attention-  practice MANY times until you can do it. 

RIGHT HAND comes out from behind your back yell GO!
RIGHT HAND comes up to forehead and Right foot snaps back to your left foot to a salute yell Cardinals!
Hand comes down from Salute to Attention Position yell GO! Then tuck on the boom.

STEP 3: Marking Time (Marching in Place)-  Watch and practice several times.

Your feet should be together-you will get a count  to start.
After the count on BEAT 1 you will LIFT YOUR LEFT HEAL A LITTLE OFF THE GROUND. Your toe will stay down like it's nailed to the floor.
On BEAT 2 your LIFT YOUR RIGHT HEAL A LITTLE OFF THE GROUND.  Your toe will stay down like its nailed to the floor.
You continue this until you are told to move ahead.  Go to horn tuck when you move ahead.

Step 4: Putting it all together! Do this AFTER YOU HAVE MASTERED STEPS 1, 2 & 3.  
Look straight ahead. When you pick up your foot to move ahead you always move ahead starting with your LEFT FOOT.  Watch and practice several times until you can do it correctly EVERY TIME.

Congratulations!  You now know how to March!  Now send me a video of you performing Rest, and Attention!  

Photo from switz1873