Definitely good advice, to stitch out all designs on sample piece first. T-shirts and stretchy fabrics can be very tricky to successfully embroider. First, make sure they are firm in the hoop but not stretched. This causes puckers. I use adhesive back stabilizer, but iron on would also be good. Both adhere to the knit and prevent it from pulling when hooped. I also like to use a water soluble stabilizer on top. Sometimes, I don't even hoop the shirt, I use the adhesive stabilizer in the hoop and float/stick the shirt to the stabilizer. This also helps prevent stretching.
The underscore (underline) should be your space. Again, try this on a test piece.
I have an older Brother embroidery machine and you can adjust where the pattern starts. And, you can preview the area it will stitch. Meaning, the machine shows you the outline of where it will stitch by moving the needle around in a square without stitching. All of this is in the operation manual for my machine. Maybe yours has it, too? Offhand, I don't recall the sequence of buttons to use for this. If you can't find it in your book, send me a PM and I will sit down with the machine so I can tell you how mine works.