Install bitcoin, install the counterparty client, place an order:

C:\counterpartyd>python counterpartyd.py order
usage: counterpartyd order [-h] --from SOURCE --get-quantity GET_QUANTITY
                           --get-asset GET_ASSET --give-quantity GIVE_QUANTITY
                           --give-asset GIVE_ASSET --expiration EXPIRATION
                           [--fee_required FEE_REQUIRED]
                           [--fee_provided FEE_PROVIDED]

GET_ASSET would be XCP and GIVE_ASSET would be BTC if buying XCP. Expiration is in blocks.

Submit, and wait for an order match. Then use btcpay to pay for the order.

There, completely trustless and decentralized.