Sheets or roll? Either way, running dead straight and square is usually more aesthetically pleasing than it is absolutely necessary. As long as the media stays under the feed rollers and the image fits onto the media, it really doesn't matter all that much. Within reason, as with all things.
If you're running rolls, stick a few inches through the feed rollers and then lock the little locker widget on the right side, from the front, of the first media roller. Then gently pull the media against the locked roller and it will square itself up reasonably well. keeping tension on the media, lower the feed rollers. It's a learned touch, keep at it and it'll become second nature to you.
With sheets, stick the sheet in and by hand run it until it's hanging there half in the back, half in the front. Then align the right edge with a handy pair of those little rectangles that are cut out of the front bezel. The smaller the gap between the media edge and the rectangle's edge the more accurately you can align things. Lower the feed rollers an press f3, Sheet. Watch the media run out against the alignment rectangles on the right. If it's unstatisfactory, do it again. Like setting up roll media, it's a learned touch.