There are a number of different ways and opinions. Here is what we recommend based off of what the Japanese Tea Instructors Association recommends:
Very basically, you should have 1 gram of leaf for every 10 ml, so if using 5 grams, use 50 ml (1.7 fl oz). 40-60˚C degrees (100-140F).
This is just for the first steep, to get a concentrated infusion of the savory umami flavor. The lower temperature draws out less, so you steep it longer, the higher temperature draws out more and faster, but you start to get catechin which adds the bitterness of sencha.
When you get really good at this, you will be able to put any amount of leaf, and know just how much water you should have (about x2 the height of the leaf if using a small houhin tea pot), and the steep will be ready, when the level of the infused leaves becomes about even with the level of the water. This takes practice.
After this first steep, the catechin is still present in the leaf. Steep very quickly, 5-10 sec., at 80-90 C degrees. This is because the leaves will be ready to release the rest of their essence. And you will get enough of a flavor without steeping longer. Then you still have enough flavor in your leaf for another two steeps depending on the quality of the leaf.
Hope this helps!