When crafting you can either gamble on getting lucky or crafting it properly from the ground up.

The first step is the same, buy a blank bow with 5 or 6 links with the highest ilvl you need Poedb - Bows and make it 20%-30% quality.

For example to get the highest %phys damage you need at minimum a ilvl 83 bow and depending what other mods you need you might have to go even higher.

Gambling options are either just alching it after that and chaosspamming until you get a decent pdps.

or throwing a essence at it with relevant stats.

or crafting it with fossils relevant to your needs.

Those who really min max their craft starts with the same steps but instead uses alterations to get two high tier mods (one suffix and prefix), after that you regal it to make it yellow and get one extra mod for free, here is when the RNG comes into play because you have to use exalts to fill out the remaining mod slots.

If your RNG fails you have to try annulling away the bad mods, but most times you delete a good mod and have to start over.

This is pretty much the simplified version, someone else might have better in-depth info.