my qstn is how to get draw whn we have 1 pawn less ?

I'm not an expert, but the basic premise of drawing a "pawn down" endgame would be putting yourself into a position where you can exchange down to "insufficient material" to mate.

Take this position from a recent game of mine:

A draw was agreed because even though I was 2 pawns up, my king is shut out of play, and there was not going to be any way for me to get my pawns down the board safely. Rook endings are notorious for being drawn with 1 and in cases like this.. even 2 pawn advantages. Other endings follow similar patterns. The player who is down 1 pawn must keep a defensive position so that he can exchange down to insufficient material. Ultimately, you have to know K+P vs K endings. If you haven't studied these, they are trickier than you might think. Knowledge of how to draw a K+P vs K ending is very good knowledge.