Make the line as short as possible and then fold it in half each time until you have a length of about 1 cm or 25 - 30 and then tie a loose knot in the middle.
Been doing that on numerous tents and awnings for decades, seems to be reliable method in my experience, and no fancy 'boy scout' knots to learn/remember/forget!
There are strings cords and ropes. No one uses string as guy lines. Nylon and polyester tunnel and dome tents use nylon cord as guy lines, sometimes sold as para chord. Big canvas patrol ridge tents and bell tents use hemp rope.
Ridge tents have main guy lines coming from the tops of the upright poles with a dolly or ornament sitting on the pole spike. These guy lines may be temporarily pegged outwards from the tent as the tent is erected but later turned inwards or storm lashed. When the tent is taken down shorten these main guy lines with the adjusters then fold in half a couple of times and tie a simple thumb knot.
Treat the shorter guy lines that hold out the walls in the same way.
Tunnel and dome tents don't have main guy lines, They do have short cord guy lines shaping the tent and holding out the walls. When you take the tent down these guy lines are usually the right length for when you put the tent back up. Don't bother shortening, folding or tying them, just make sure that as you fold and roll the tent they are on the inside of the package and they'll be all right when you come to unroll and unfold the tent.
Army and Scout tents are repacked according to a set of rules because it is unlikely the same people will use that tent next time and the next people to use that tent will want to find it packed in a standard way. Your family or personal tent will be put up by you next time. So long as it is dry, packs into the tent bag and comes out untangled next time it's up to you how you do it. Don't worry too much about it.
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