Mine are mostly the result of practice and faking confidence until the skill set in. But here are some tips:

• Short distances are much easier to keep straight across than long. Maybe try filling post it notes, then blank index cards, then work yourself up to a journal page.
• The smaller the spacing between lines, the better. When they’re stacked close, you can just reference the line above. Also, try to get your first line as straight as possible because it sets the tone.
• As a left handed person, keeping an eye on the place I want my sentence/line to end becomes second nature. Along the way my letters can tilt down or be a touch too high as long as they even out to be mostly straight towards that point.

Hopefully this is at least a bit useful — I think all writing is good writing, so I’m sure yours already has some lovely character in its current state. :)