Placing the long line forehand at the right time is a formidable weapon; you can put your opponent in great difficulty.
To practice making long line forehands, I set up a very easy exercise with the ball machine.

Exercise 1: Hitting a long line forehand

I put the low machine in place and I make it send me a ball on my forehand. (I reproduce a cross forehand from an opponent)
I know where the ball is going to land so I position myself and practice hitting my long line forehand.
I will be able to test, open support/line support/semi-open...
I think in any case that it is good to train with all the supports because in a match we can find ourselves in all game situations.

With this exercise I am mainly trying to adjust my stroke and improve my hitting. Right now I'm not focusing on traveling.

Exercise 2: Position yourself and hit a long line forehand.

It's the same exercise except that this time I'm going to add the difficulty of moving, in fact, in a real situation, initially I'm not positioned where the ball is going to come, so to get as close as possible, I'm going to position myself where I would have been placed if I had hit a ball towards where the machine "returns" my shot.
So I'm going to start positioning myself a little to the right of the T, and to return the ball I'm going to have to run, adjust my position with small steps... then hit the ball long line.
This exercise corresponds to a more real game situation.

TIPS: Ball machines have a small flaw: we cannot visualize the moment when the opponent (the machine) hits the ball. This distorts the game to the extent that we don't know when to do our support resumption... On my machine, I have a little auditory cue which allows me to do my support resumption at the right time...
It's up to you to find what you can do on your machine to get this "resumption of support" mark.

Download this sheet in PDF format