There are two common types of local modifications that can be done very simply. Do not forget that documents produced using such modifications will not be usable at other places (such documents are called `non-portable').
One type of modification is the use of personal commands for commonly
used symbols or constructions. These should be put into a package
file (for example, one called mymacros.sty) and loaded by
putting \usepackage{mymacros} in the document preamble.
Another type is a local document class that is very similar to one of the standard classes but contains some straightforward modifications such as extra environments, different values for some parameters, etc. These should be put into a class file; here we shall describe a simple method of constructing such a file using, as an example, a class called larticle that is very similar to the article class.
The class file called larticle.cls should (after the preliminary identification commands) start as follows:
\LoadClassWithOptions{article}
This command should be followed by whatever additions and changes you
wish to make to the results of reading in the file
article.sty.
The effect of using the above \LoadClassWithOptions command is to
load the standard class file article with whatever options
are asked for by the document. Thus a document using your
larticle class can specify any option that could be specified
when using the standard article class; for example:
\documentclass[a4paper,twocolumn,dvips]{larticle}