Casting on to continue knitting

I have a problem I have yet to fix. I have trouble when I have to cast on to something in the middle of knitting it. I am getting ready to make a cropped jacket for my sister that is one piece and it has some sections where I have to cast on to increase the stitches. On other projects, I always have a hole between the knitted piece to the new cast on. What can I do to keep this from happening? Which cast on method should I use? I use a weird modified knitting on casting on when I start a project so I haven’t really used anything else. Thanks for your suggestions. I need to knit a piece and use a cast on method to practice this before I start the jacket! lol

One thing you can try is CO an extra, and then knit tog one of the new sts with a previous stitch.

I use the cable cast on method (
I’m knitting a shrug right now, and that’s what I used to add stitches for the sleeves.