Step one: a few weeks before hand, ask the congregation to return the palm crosses they were given last year on Palm Sunday so that they can be burnt for use on Ash Wednesday. This has a double benefit: it gets you a supply of palm crosses to burn and involves everyone in what is happening through supplying the raw ingredient.
Step two: find a sheltered spot - I use the barbecue in the back garden. (I have done this in the front garden but it took some explaining to the postman as to why a vicar was burning crosses outside the vicarage!) Take a metal bowl (tip: don't use the best mixing bowl from the kitchen to avoid serious domestic strife). Pile up the palm crosses and burn them. If they are large, it might work better if they are cut up a little first, but if they are dry and it is not a windy day, they should burn pretty well as they are.
Step three: use a long gas/oil fire-lighter and set fire to them. A blow torch will do the job even better. The point is you may need to light them several times or keep the flame there for a while until they catch. Palm crosses can be notoriously difficult to set fire to, but once the fire gets going they burn well. You might need some barbecue tongues to move them around a bit so that all of them burn.
Step four: allow to cool!
Step five: spoon some of the ash into a small bowl and chop with a fairly sharp spoon. The aim is to reduce the ash to a fine powder, or fairly close to that. Rapid but gentle chopping movements work well and it will take several minutes to achieve the grade of ash you are after.
"Remember that you are dust, and to dust you shall return.
Turn away from sin and be faithful to Christ."
This act is very powerful - for those receiving and those administering it. It gives pastoral and sacramental ministry an edge. It makes it real; all pretence is stripped away and we are all confronted with the raw reality that we are fragile human beings before God. We get it right and we get it wrong. We are caught up in all sorts of complications, some of which are beyond us to sort out. We trust in God's redeeming grace to bring all of this through to resolution. We will die one day and our hope is in the loving mercy of the God who gave us life and will through Jesus Christ bring us to share in the life of his eternity.