How can I play StarCraft 2 competitively against less skilled friends?
Solution 1:
You can play with low-level computers on your team vs. your friends. I am not a big fan of uneven teams insofar as having a different number of players on the teams. A good compromise is giving your team an easy or medium computer, depending on your friends' skill levels, to help compensate. You cannot coordinate with your ally while they can coordinate with each other.
If you want to play with only one friend, you can alter this idea by giving him a hard or insane ally while you still have your easy or medium ally. Experiment with the difficulties until you find something manageable and challenging for both sides.
Solution 2:
If you're willing to put some time into developing a custom map, here are some ideas:
Player A gets a high yield base to start, Player B gets a base with only 1 gas.
Player A gets an easy to defend base to start, Player B's base is in a pit surrounded by high ground.
Player A gets X minerals/gas per minute, automatically.
Player B has a artificial cap on number of workers and/or food.
Player A gets some wacky unit like the Laser Drill from the campaign, whose damage or recharge rate or something else can be adjusted to make the difficulty even out.
Player A gets full vision of the map.
UPDATE: OP wants to keep the lower skill player's game experience "normal" so they get meaningful practice. with that in mind:
Your first expansion is hard to defend - wide open, with overlooking cliffs. Their natural is easily defended, to encourage expanding before their main runs out of minerals, a common beginner problem.
Your units cost more food (or your supply units provide less pop)? this would be more "normal" than reducing the damage or HP of your units. So your buddy will still learn that X marines beat Y zealots, but you'd have less zealots. this would of course run the risk of your instincts getting confused when you switch back to regular games.
Solution 3:
One of the best handicaps in my opinion is to hide the minimap entirely with a Post-It note.
People really forget how important the minimap is, until it is gone.
It feels as if you are blind. Your reaction is really late to attacks and you cannot see drops. Your only option is to hit space to jump to the attack.
Solution 4:
You use a stopwatch and give him a 30 (45? 60? 90?) second head start.