How to advance lower characters in Shadowland?

Solution 1:

The simplest way is to start the Battle for Azeroth campaign:

• For Alliance, head to Stormwind, talk to the Hero's Herald in the center of the Trade District, and pick up the quest "Tides of War".
• For Horde, head to Orgrimmar, talk to the Warchief's Herald near the stronghold, and pick up the quest "Mission Statement".

Quest zones throughout Battle for Azeroth will scale to your level from 10-50, so just keep following the breadcrumbs.

If you've already completed Battle for Azeroth at least once, another option is to use Chromie Time. Visit Chromie near the Horde or Alliance Embassies in their respective capital cities and you can activate this feature to allow your current character to level through a previous expansion of your choice. While you're in Chromie Time, the chosen expansion's zones will also scale with you from 10-50.

You can also level through group content with the Group Finder (I) or unranked PvP (H).

However you choose to level, once you hit level 48 the quest "Shadowlands: A Chilling Summons" should be automatically added to your quest log. (If it isn't, you can generally prompt it by changing zones.)

Solution 2:

Cadence already hit the nail on the head: the default way to level 10-50 is to play BfA, the alternate way is to speak to Chromie and then play through one of the older expansions. While any one of them will easily get you enough XP to hit level 50, some may get you there faster than others. PCGamer has a leveling guide, as do Wowhead and digitaltrends (Warlords of Draenor seems to be the fastest, followed by Legion).