Each project that announces the sale of ICO will always have the accompanying purchase guide. Depending on the organization of each project, the purchase method is different.
Since most ICOs currently use blockchain Ethereum, many of these instructions will focus on how to purchase ERC-20 tokens.
Projects running on NEO, QTUM, ICON ... will be updated in the coming time.
Often, projects running on Ethereum's platform, there will be a way to purchase the ICO as follows:
Option 1:
Option 2: