The world of indexers is small but there are several professional indexing organizations in this world. Even though anyone in the world with a computer could provide you with an "index" it is advisable to seek someone in your own country who truly understands your language, its quirks, and the word choices someone living there might use.
Also try to find someone who specializes in your field. The American Society for Indexing has many Special Interest Groups (SIGs) comprised of indexers who have become experts in those knowledge arenas. These groups can be accessed through the ASI site.
INDEXING GROUPS
American Society For Indexing (ASI): www.asindexing.org
Australian And New Zealand Society Of Indexers (ANZSI): www.anzsi.org
China Society Of Indexers (CSI): www.cnindex.fudan.edu.cn
Deutsches Netzwerk Der Indeser (DNI) (German Network Of Indexers): www.d-indexer.org/welcome.html
Indexing Society Of Canada/Societe Canadienne D'Indexation (ISC/SCI): www.indexers.ca
Nederlands Indexers Netwerk (NIN) (Netherlands Indexing Network): www.indexers.nl/index_uk.html
Society Of Indexers (SI): www.inderers.org.uk
There are several courses available that one can take to learn indexing "best practices." Some are through universities, many are given by freelance indexers.
INDEXING COURSES
American Society for Indexing: http://www.asindexing.org/i4a/pages/index.cfm?pageid=3444
University of California Berkeley: http://extension.berkeley.edu/catalog/course394.html
Even though one can generate an index using index cards (the Dark Ages) or any word processing software (adequate), professional indexers of today use one of three software programs designed specifically for the indexing process. Each is distinctive from the other two. Before investing in one try their free trial versions.
INDEXING SOFTWARE
SKY: http://www.sky-software.com/
Cindex: http://indexres.com/home.php
Macrex: http://www.macrex.com/
EMBEDDED INDEXING SOFTWARE
InDesign by Adobe: https://www.adobe.com/products/indesign.html
Index Manager by Klarso: https://index-manager.net/en/indexer-en/?v=7516fd43adaa
PubIndex by PubHub Tech: https://www.pubsuite.tech/pubindex/index.php?route=common/home
Word by Microsoft: https://www.microsoft.com/en-us/microsoft-365/word