Old house...upstairs studio...good luck with blocking the sound from going downstairs or any other way without 1.) ripping open the existing walls/floor/ceiling and doing some serious soundproofing, or 2) adding a lot of layers sheet-rock layers to the existing walls/floor/ceiling or 3) floating a new room inside that room.
It really comes down to how much the sound needs to be blocked,,,and how much work/cost you want to throw at it...?
Upstairs floors are tricky, because they are tied to the walls, which will transmit down/up, and they are also the downstairs ceilings.
Old...even newer houses...rarely have even the most basic fiberglass insulation between inside walls/floors/ceilings.
If you're really looking for more serious soundproofing...you may want to *start* by ripping out the existing wall surface inside the room, so you are down to the exposed beams in your studio space...and load them up with insulation...then do a couple/three layers of sheet-rock to close them, with glue and screws. You may also want some kind of damper between the sheet-rock and the studs.
For the floor...if it's wood, it might be easier to open the ceiling below, if it's sheet-rock...and work from there.
In the end....everything is still coupled together at the corners and at the joints...so you'll have to see what you end up with....and then you may need to do some more.
That is IMO, the "easiest" DIY approach...without getting into more serious demolition and rebuilding, but again...really weigh out how much soundproofing is needed...just a little less noise or almost completely quiet in the rest of the house.
Is this for like a full-tilt studio...or a small hobby thing?
Also.,..will there be drums and loud guitar amps...or mostly quieter stuff, and lots of synths and samples?