Short version, prepare it all together in one bottle at room temperature, non-sterile. The agarose won't dissolve at RT and that's ok, when you sterilize the media the agarose will melt and dissolve. The goal is to make sure the MS powder is mostly dissolved. If you can, pH adjust to 5.6-5.7. If you're in the kitchen, it might be a good idea to microwave until the agar melts so you can aliquot your media into final vessels and then pressure cook/autoclave. Usually, since I live in a lab and it's easy to maintain sterility, I'm lazy and just autoclave the media bottle(s) and aliquot afterwards (so I can skip the microwave step). And you can definitely do this too, but if you're in the kitchen, every time you open anything the odds of contamination are much higher so the microwave step might be worth it.
If you're in a lab setting, the optimal way to prepare media with heat-sensitive additives is to get media without those additives, (e.g. plain MS + vit) and filter-sterilize concentrated solutions of whatever additives you want. So you would be able to prepare and autoclave MS + vit + agar, let it cool a bit, then sterilely add whatever small volumes (because you made concentrated stock solutions) of hormones and C sources, mix, and then pour your plates. But that assumes access to lab stuff.
Sorry that got long winded. But media prep is basically all I do lately so I thought I'd share
Murashige and Skoog media preparation
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