When you say partner i hope you ain't talking about marriage, for marriage go for marriage counseling...... And leave musing out of it.

There is actually no best way i mean it is called break up. However if both partner are on the same page it is easier...

So take your time sit your partner down and tell him/her how things are obviously not working out. Its important you put the feelings of you partner into consideration so you don't sound selfish, show them you genuinely care for them but you don't see a future where both of you can feature.

Again, it is not going to be easy but you have got to do what you have got to do.