EASY- get under the house with a floor jack, some 4x4's, a sheet of 1" plywood to set the floor jack on (doesn't have to be big, just enough to bear the floor jack and spread the load some.) You will also need some 2x8's, and some pier blocks. The 4x4's should be pressure treated.
(I am making a lot of assumptions about your crawlspace, including that you have one, so adjust this to fit your situation)
After you have located the area where the joists are sagging-settling figure out a place to support them with a couple of sistered up 2x8's. You are going to need help for this. Support the "beam" (2-2x8's) where you want it, and put the floor jack in the middle underneath it. Cut a 4x4 to fit between the jack and the beam, and SLOWLY lift the beam until the joists are supported about where you want them. (You will need someone upstairs to let you know you've accomplished this.) Once you get it where you want it, go a tad more (1/8" or so) to allow for settling of your new support.
Place a Pier block on each end of the beam and cut 4x4's to fit the space between the blocks and the beam, jacking the beam an additional 1/8" to allow you to insert the posts.
Slowly lower the jack to apply pressure to the beam on the new posts, then nail it all together with vinyl coated sinkers. If it makes you feel better, Simpson makes terrific steel connectors for all of these joints. Use joist hanger nails for the Simpson connections, they are rated for it (the heads won't pop off).
You may find that doing several short spans like this will be easiest, and I wouldn't recommend doing any span in excess of about 8'. If any of your pierblock to beam distances exceed 4', I would recommend the simpsons for sure.
Keep in mind that jacking up your house is going to do all kinds of interesting things to it, which is why you want to proceed with caution. You can make it so doors and windows won't open or close, you can crack plaster in new and exciting ways, and give yourself lots of reasons to buy excedrine. You might consider getting some estimates from some local contractors, and letting them deal with the headaches...
Remember that just because you have concrete under your house, doesn't mean it is very strong. In 1925, and before, rebar was a foreign object in a residential foundation, and the quality of the concrete itself was marginal at best. Many foundations lack a footing, which is unheard of today.
Lastly, and I have to mention this, water penetration into any part of your foundation, crawlspace, etc will not only rot your structural members, but it undermines the soil your foundation is sitting on, and that can be the cause of your sagging floors. Make sure your rainwater is directed AWAY from your home and foundation, with quality gutters, slopage (your landscaping is graded away from the house) and that no water is allowed to puddle next to the foundation.
Be very very careful when you approach this project, and do it slowly with your eyes wide open.