For in game instructions, just launch the game and tap on the Instructions button.
Gameplay is deceptively simple. The game board is a 9 x 9 grid. The goal is to fill each of the 81 squares with the numbers 1 thru 9, but all of the numbers in each row and column must be unique to that row or column. In addition, the game board is divided into 9, 3 x 3 sub-sections. The numbers to each of these sections must also be unique.
Numbers can be entered manually - tap on a cell then simply use your finger to write the number on screen. Tap the Done button to enter that number into the cell. Numbers can also be entered using a number pad. This behavior can be set by tapping the Settings button at the main screen.