A basement ceiling in an older house will often not be level. The joists settle as people walk over them. There is not a lot you can do about the problem except to level it by installing a dropped ceiling. A dropped ceiling is fiddly, but not difficult to install. It just takes some time. Follow these steps to level a ceiling with a crop ceiling system.
Step 1 - Mark the Perimeter Walls
Mark the walls at least 4-inches down from the joists. Chalk-line all the perimeter walls. Use the laser level to make sure they are all even and horizontal.
Step 2 - Staple the Wall Angles on Perimeter Walls
Place a nail on one of the corner marks and loosely rest the wall angle strip on it. Staple the wall angles on all the perimeter walls of the area you will tile.
Step 3 - Find the Number of Main Tees Needed
Measure the width of the basement ceiling along the floor joists. Divide this number by 2 to find the number of main tees needed. Make a mark at one corner, 24-inches from the wall, and across the floor joists. Drive an anchor. At least 1-inch of the anchor must be showing. Nail a piece of 2x4-inch wood, as needed, if you end up between joists. Repeat this step at the other side of the wall.
Step 4 - Tie the Tees to the Anchors
Run a string from one anchor to the other to use as a reference line for the tees. Making sure the tees are level, tie the first row of tees to the anchors with wire every 24-inches for 2x2-inch tiles. If you want 2x4-inch tiles, place the cross tees every 48-inches.
Step 5 - Install the Cross Tees
Install the cross tees between the wall angles and the main tees. The cross tee should be cut at the wall angel.
Step 6 - Install the Basement Ceiling Tiles
After you have placed all the cross tees along the main tee, slide the basement ceiling tiles into place, 1 row at a time. Check that they are level as you go.
You can use drywall panels cut to size a bit less than 24-square inches. This method is cheaper than using acoustic tiles. For a decorative effect, stipple or paint the drywall panels before you cut them. You may paint or stencil tiles to create an ornate look and dress up the ceiling.
If the basement ceiling is very uneven and you can even feel it on the floor above, you should find the reason for the problem before you try to fix it. If you have no other choice and only 1 or 2 joists are involved, you can support them with posts. These posts can be made quite decorative, especially if you use carved veranda posts.