All info is here: https://www.fbi.gov/about-us/cjis/identity-history-summary-checks/submitting-an-identity-history-summary-request-to-the-fbi
But here is a short summary:
1) Download the checklist (https://www.fbi.gov/about-us/cjis/identity-history-summary-checks/identity-history-summary-request-checklist-1)
2) Download the application form (https://forms.fbi.gov/identity-history-summary-checks-review) and fill it out
3) Go get your fingerprints done on a FD-258 card. The cheapest option is your local police station. You can pay more at private fingerprinting places. If it hard to get good prints for you (dry/cracked hands, poor ridges, etc), it might be better to a private place because they have better equipment and have a stock of blank FD-258 cards. Where you got to get your prints done, you will need to show some government ID like passport or driver's license. Look at the checklist to see what the requirements are for the FD-258 card.
4) Figure out if you are going to pay the $18 (per copy) fee by certified check / money order or credit card. If it is the latter, download the credit card payment form (https://www.fbi.gov/about-us/cjis/identity-history-summary-checks/credit-card-payment-form) and fill it out. If by money order / certified check, make sure it is payable to "Treasury of the United States".
5) create a document that has your contact info (name, address, phone number, e-mail) and print it
6) Check the checklist to make sure you have everything. Check the boxes in the checklist
7) Get an envelope and put your completed application form, completed FD-258, payment (certified check or money order or credit card payment form), the completed checklist form, and contact document in it
8 ) mail via priority (trackable) to :
FBI CJIS Division – Summary Request
1000 Custer Hollow Road
Clarksburg, WV 26306
in any case, carefully follow the checklist. It has a summary of all the steps you need to know.
good luck