Soft-walled aboveground pools, especially vinyl models, primarily only use ladders that fit over the pool wall and require no installation. Those pool ladders that require installation and attach to the pool are only recommended for hard-walled aboveground swimming pools. Once the ladder is assembled, installation primarily involves placing the ladder in the pool and securing it to the pool's top rail with a few bolts. You may need to pre-drill the holes beforehand. Specific installation instructions will vary with each model.
Place the ladder in the pool so that the support legs are on the outside of the pool wall. Set the ladder on a solid base, over an upright support on the pool wall, if possible. Once placed in the pool, the ladder may have adjustable tubes you can move to make it fit more securely up against the pool.
Align the holes in the platform, if your ladder model attaches to the pool, with the pool's top rail.
Mark where to drill the holes, based on where the holes line up with the pool.
Drill holes in the pool's top rail, based on the size of the bolts. You will probably need about a 1/4-inch bit, but verify the size before drilling.
Place the bolts through the pre-drilled holes in both the ladder and the pool's top rail. Secure the bolts with the washer and nut, if included with your ladder model.