Border help please

Hi. I am knitting an Afghan in garter stitch (that’s what was requested). However I need a border for it. In making a sample swatch to work it out, I have tried a simple seed stitch (6 on each side) but I can’t get the border even on each side. One side has five columns, the other 6. I know it has to do with the garter stitch but can’t figure out how to compensate. Please help. Thanks.

Welcome to KH!
Are you going to knit the seed stitch border at the same time as the garter stitch or add it on later?
Assuming it’s knit at the same time, I’m not sure that I understand what you mean by one side has 5 sts and the other side 6 sts? Place markers for the border, 6sts on each side and the columns of seed stitch should maintain at 6sts each.
You may need to work an extra two rows of garter every 20 ridges or so because garter stitch rows will be shorter than seed stitch.