When it comes time to make an offer to purchase a property there are two general ways to go about it. One way is formally in writing. The other way is verbally, followed by a written offer.
Written is strongest
The strongest way to make an offer is to put it in writing in the form of a contract, along with a down payment to be held by an escrow intermediary (attorney, real estate agent, title company, etc.). However, some people prefer to make a verbal offer. Once there is general agreement on price and other major issues, put it in writing for signature by the parties.
Let circumstances dictate
How you do it depends on the circumstances and personalities involved. For example, a reluctant seller may be persuaded by a strong written offer that includes a large down payment. Or, if a seller and buyer live in different states, it may be more practical to make a verbal offer first, then get it in writing after key agreements are reached.
As in many things, let circumstances and common sense be your guide.