How can I even out my subfloor?
It would be a straightforward process to shim all the low joists along their lengths. One would go about this by finding the highest joists and then shimming all the rest to be even with those highest joists. Of course this requires removing all of the existing subflooring.

It would require also that the highest joists be checked lengthwise to see if their top sides are straight. They may be high because they have bowed up during their aging. If they have bowed up then it would be necessary to adjust those joists first before tackling the lower joists. 

There are multiple techniques to find the shim up amounts for each joist. Probably the easiest is to place long known straight members across the high joists at right angles. The bottom of the straight member represents the top edge of the shim that needs to be installed.

Another technique is to use a nylon string line stretched across the top of the joists and having its height adjusted so that it just skims the top of the highest joists. The string can be attached to vertical 1"x2" boards that are temporarily nailed or screwed to the face of the outermost joists across the room.

It would require multiple strings or straight members across the length span of the joists to get the reference level for each shim.

A really good way to make the shims is to acquire nice straight members such as 2"x4"s (or 2x6s) that are the same thickness as the joists themselves. One by one a 2x4 can be clamped to the side of a low joist with its top side adjusted to the string or cross reference straight member. Once carefully adjusted and clamped in place the 2x4 can be scribed with a line that follows the very top of the existing joist. A power saw is then used to rip the shim from the 2x4 to make the shim piece that attaches to the top of the joist. (Take extra care in cutting carefully and along the correct side of the line). The shims can be installed in place with construction adhesive and small finish nails or pneumatically shot brads.

The straight 2x4 material can be used to make two shims from the two straight edges. A more economical way is to use just one side of the 2x4 and then run the remaining piece through a table saw with the straight edge against the fence to restore the piece to two straight parallel edges to get it ready for the next shim marking.