You can do it now(ish).
Ideally you would do the craft when the root stock is showing signs of 'waking up', buds are just about starting to swell, but no means of ready to break up yet.
And the graft wood still dormant.
To get into this stage, I had my root stocks potted up in cold GH they were that bit of more sheltered and wanted to get on with a spring. The scions were kept in storage in fridge in cold or I took fresh wood from trees outside that was still totally in 'hibernation'.
Once you join both bits up...the roots stock already has moisture moving up being 'woken up'..so it is able to keep the union moist and to start immediate 'healing'.
If you are going to do you crafting outdoors..wait till the rootstock is ready, but in mean while, you can gather you graft wood, budle them up and keep it wrapped in polythene in fridge until you are ready for the action.
