Just like anything else, really---start at the beginning and master the basics.
In this case, learn to fly straight lines at a constant depth and height---not as easy as it sounds!
If you have a 2 metre plane, mark out a line 150 metres out(or a little more)-----------if you have a 90-sizes plane ,a little less---say 120metres.
mark out the 60 degree box, with a centre line where you stand to fly, and two 60 degree lines leaving it----parcel tape works well.
Then practice flying up and down the 150 metre line, on the base line to begin with, using something like a stall turn to reverse direction at each end.
That's the beginning----good luck!