Classic to Lightning Migration

• First step for Lightning Migration or a self guide is to run the Lightning Experience Readiness check which is available in Lightning Experience Transition Assistant from setup menu -

https://help.salesforce.com/articleView?id=lex_readiness_check.htm&type=5

• If you have Visualforce pages, Javascript buttons and links, or Quick Actions buttons and links, run the Lightning Experience Configuration Converter to get a more detailed analysis of those feature areas.

• Select a user from the "Ready" or "Almost Ready" profiles as your transition champion. A champion should be a user who is quick to adopt new technologies, willing to test beloved Salesforce Classic features in Lightning Experience, and who will be vocal about offering recommendations on tweaking Lightning Experience for your org.  

• Turn on Lightning Experience, and give your champion access to the new interface. Read up on what enabling Lightning Experience entails before you flip the switch. For example, standard profile users automatically get access to Lightning Experience when it's enabled, so move them to a custom profile until you're ready for them to use the new interface.  

• Open the Lightning Experience Configuration Converter for JavaScript Buttons/VF pages to scan your org for buttons and links/pages and get recommendations

Follow the below trailheads so you get hands on and have an effective transition plan -

https://trailhead.salesforce.com/en/content/learn/trails/lex_admin_migration
-- Getting Started with Lightning Experience - https://trailhead.salesforce.com/trails/lex_admin_implementation
-- Make the Move to Lightning Experience - https://trailhead.salesforce.com/trails/lex_admin_migration
-- Lightning Alternatives to JavaScript Buttons - https://trailhead.salesforce.com/en/modules/lex_javascript_button_migration
-- Optimize Sales Features for Lightning Experience - https://trailhead.salesforce.com/trails/sales_admin
-- Sell Lightning Fast with Sales Cloud - https://trailhead.salesforce.com/trails/lex_end_user
-- Lightning Experience Specialist Superbadge - https://trailhead.salesforce.com/super_badges/superbadge_lex
-- Develop for Lightning Experience - https://trailhead.salesforce.com/trails/lex_dev

VF  to Lightning Migration -
https://developer.salesforce.com/blogs/2018/01/making-visualforce-work-lightning.html