Determine the gear stats and armor type you need for your new character. For example, a paladin requires Strength as their main stat, and heavy armor (the armor type) will scale as the character scales if they are initially unable to wear heavy armor due to being a lower level. Your optimal heirloom gear set will feature the same main stat as your starting gear.
I'd rather not have to manually re-do all the settings for all my addons in-game unless it's absolutely necessary (since I don't have memorized what the exact settings are I use, so I'd have to take screenshots of them all and then remake the settings to match the screenshots). So I'd rather get some more input about this problem before I try troubleshooting it that way.

Most heirlooms only work in a limited level range in their default form and must be upgraded to work at higher levels. That costs gold, and you’ll need around 30,000 gold to outfit yourself with a fully upgraded set (counting only those that give you bonus experience). You may need to spend some time on a boosted character to acquire the gold you need to outfit another character with heirlooms, but it’s well worth the effort.
Never ever pay for leveling guides. Want/need to learn about and use. Dugi is hosting a contest where you can get. Saharun is one of croatia's loveliest but least known spots (picture: alamy). Open my map to see where the nearest quest takes place and go there, through the questtracking you can quickly see what is to be expected of you or even when you look at your tooltip while hovering over mobs. It sounds unrealistic but the video promotion of the guide clearly shows how you can do it. Unfortunately it is a website and you may have to tab out if you aren't sporting a multi-monitor set up.