To keep things cheap, first I would add some 2x4 across (or 2x-whatever-the-height-of-those-beams-is), between each beam, so you will create "squares" that need to be filled. In those squares you can add thick pieces of styrofoam (link to styrofoam at the bottom, I just did a quick search on Amazon for you to get an idea). You could buy a big sheet, and cut each square to size. Styrofoam usually comes in like 1" or 2" thick. You want to cover the full depth, so you may have to put two or three layers of it in each square.
Once you got that done, it's time to add the rubber/ rubber pads on the wood, on which the floor (piece of plywood) will rest. Those rubber pads are expensive (because they have to be extremely sound-absorbent + really strong, to carry the weight of a full floor). In your case, you may be better off getting something like a baby mat (link at the bottom of this message), and cutting it up and either gluing or screwing the "strings" of rubber to the wood.
Now, there are different ways to keep going. My recommendation is: cut a piece of plywood the size of the booth, minus 1 inch, glue some rug on it, or something that makes it feel nice (let's face it, plywood is ugly). Wait for it to dry. When you put the plywood on the floor, you'll have a gap of half inch on all sides (because you cut it 1 inch smaller). Fit yet another piece of baby mat in all those gaps around your floor. What you are creating here is a floating floor that has zero hard connection to anything (between the beams and the floor you have baby mat, and between the floor and the walls, you also have baby mat!). Once this is set, you can either leave it like that (don't screw anything, the floor won't move unless your daughter is recording and tap dancing like fire at the same time), oooor you can screw the floor to the walls with THIS TYPE of tip-over restraint (link at the bottom; I say THIS TYPE because as you can see, it is a strong piece of fabric that connects the floor to the walls, but it's still fabric, so it is flexible and it absorbs vibrations).
As per the walls... there are many treatments you can do. Again, with a mindset of keeping it cheap, you could add blankets or rugs hanging from those walls. But put them in a way that it's not hard to take out, because they may collect dust and get moldy depending on where this booth is located. The biggest deal is the floor, and with what I told you, you'll have that covered.
In addition: your daughter may know, but she'll need a cardiod mic (condenser mics are better, but expensive; a dynamic mic will work fine), and an isolation shield like this one for the mic (link at the bottom). The isolation shield is a great noise-reduction solution for the mic, and it replaces having to install the expensive absorbing panels to the whole booth.
P.S.: damn, I love sound...
Examples of styrofoam you can get: https://www.amazon.com/s?k=styrofoam&crid=2Q55JPKNXLAHV&sprefix=styrofoam,aps,172&ref=nb_sb_noss_1
Baby matt: https://www.amazon.com/Angels-Color...8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
