Getting to Keukenhoef from Amsterdam

Just buy the bus entry combo ticket.
You can buy it and get the directions where to catch the bus online on the official website
https://keukenhof.nl/en/plan-your-visit/

The shuttle leaves from both outside the airport and central Amsterdam.

I have been to there two times. The best time to visit is around 3 pm to closing when the crowds are gone so you have space to sit and avoid arriving from 10am to 2pm.

We bought the express bus and entrance combo ticket.
https://keukenhof.nl/en/plan-your-visit/
So for us the easiest way from Amsterdam (since we had the combi ticket) is/was to take the train to Schiphol Airport.
At the airport, exit the building, turn right and walk (about 300 feet) to the end of the building where the Keukenhof and bus personnel will be waiting to check your ticket and put you on a bus that goes directly to Keukenhoef. Couldn't be easier
We took a 10 am bus, got there by 10:30 and stayed until 3:30 pm so 5 hours for us.
The return trip is similar. At the bus stop, you take the bus back to the airport. A the airport, after buying your train ticket or using your OVchipkaart you take the stairs or escalator down to the train platform and take the next train back to Amsterdam Central. So it was an all day affair for us.
Have fun. Keukenhoef is worth it.