Mix to mortar to � 1 part cement, 3 parts building sand and � part hydrated lime (this helps with movement and makes the mortar less likely to crack). Also add some PVA bonding to the gauging water.
The trick here is consistency! Make it too wet and you will struggle and get it all over the bricks, it needs to be pretty firm so that if you press the mortar against the side of the bucket you can cut it with the trowel like cutting a piece of marzipan with a knife.
The mortar should stand up on the trowel without sagging. Take your time and ensure you do not get it all over the bricks as this looks terrible. Repoint the horizontal joints then the vertical joints in the area that you have just ground out. When you get close to the next row of joints to grind out, put the cement somewhere safe.
Taken from
http://www.houseprofessionals.com/diy/brickwor k/repointing.htm