If you already have bitcoins you might do following:

The best way (or the most common) is to use wallet in your smartphone and do transaction through mobile.

1. Scan receivers QR code to your smartphone wallet
2. Send bitcoins
3. Wait until your transaction receives enough confirmations - can be cheched at www.blockchain.com just by entering your address and finding up the latest transaction. (don't get scared if the transaction doesn't show up within minutes it might get also hours depending on your transaction fees - the higher the fee is the faster confirmation proccess will be) With 0.001 BTC fee the transaction will show up in the next block.
4. Obtain bought item

In case that you don't have smartphone, bring your laptop to place, where you can acccess internet connection.

You will need to get receivers public address to send bitcoins to - with smartphone you just scan address.

Hope it helps.

If you dont have bitcoins you will need to buy some through exchange or localbitcoins.com.

I think that the best way would be for both of you to have some way of being able to send and receive bitcoins on your phone. In person you could then send eachother the coins, then once the other person gets them, then they could hand you the goods.