You need to create a psm1 file (powershell module) where you specific your dll with yours cmdlets to load and add aliases in this way:
In your module folder ( Get-ModuleFolder
give a list of all if you have more that the default one, in my example I use the first one) create a folder with same name of your .dll
and a SameNameOfYourDll.psm1 with this content:
Import-module "$((Get-ModulePath)[0])mycustomcmdlet\mycustomcmdlet.dll"
set-alias gdl Get-DirectoryListing -scope Global
For more raffinate module building look also at module manifest
Module manifest is the preffered way for .dll with custom cdmlets.