How do you guys do it if you want to level up multiple skills for a sim?

MC Commander, max all skills.

I mostly just do the skills one at a time by using paintings with whatever emotion I need and then I use that painting to keep my sim focused, self-assured etc.

With this particular iteration of the Sims, I have caved. I use MCCC and cheat up whatever skill(s) I want my Sim to have straight out of CAS. I find it particularly abhorrent when creating an entire family in CAS, that some of the kids can been teens and yet they have no skills, neither do the parents. I would love it were we to add skills, even if by random, to our Sims while still in CAS.

If a Sim does have to increase a skill(s) I still attempt to have them do it without cheats. I purposefully leave my potential celebrity Sim with a skill level below in Professional singing (since I use Road to Fame mod) and a level below in modeling. Most musicians who rise in fame grace the covers of periodicals, after all. So, I make my Sims work at the rest of their skills, despite their boost.

All at the same time for me. I like to mix things up a bit to keep it interesting. I play with aging off too, so taking it slowly with multiple skills doesn’t bother me.

I generally will do them in rotation - work on one for a while, break and take care of the sim's needs, then work on another. Lather, rinse, repeat until the skills are high enough.

I only level skills when they're required for careers. The rest level slowly by themselves or they don't level at all.

usually one skill at time if there is something I really need to teach my sim
though I usually only max more than 1 or 2 skills on my older sims that got kids
and so I can do the family aspiration and have their kids practice things in same room as them
which allows them to learn like 3 skills at once only practicing one of them ;)

I make my sims watch the cooking channel a lot whilst eating to build up their cooking skill, and jogging is a favourite for exercise. I get the Gym Bunny trait a lot so they can build fun.