Imprints are made with the Inscription skill and can be bought from other players, you do not need the Inscription skill to fit an Imprint.
Make sure you have taken the imprint you wish to fit out of your Trade Bag if you purchased it on the Marketeer or from an Auction, the item should be in your backpack.
Go to your Inventory screen, click on the imprint you wish to use to apply to a weapon and click the Use button. You wil then be taken to the inscribe imprint screen which will provide a list of all of the weapons or tools in your inventory that can have the imprint installed.
You can overwrite an existing inscribed item with another inscription of lesser or greater power. Please note however that if you try to install a more powerful spell than the item would normally allow it will adjust the item level to suit the imprint, this is to prevent lower level players from running around with high level inscriptions on their weapons.
After an item has been imprinted, you can remove the affects of the imprint at any time but you will not get the imrpint back nor will you lower the level of the item if the items level was raised due to a higher level imprint.
Pay attention when inscribing, while a higher damage inscription is nice a slightly lower damage spell that has a higher trigger percentage could end up doing far more damage because it fires more often.