I am assuming from the wording of your letter that you do not currently have to complete self assessment tax returns.
If you have a PAYE income source, and the miscellaneous earnings are modest, so that the tax can be collected by an adjustment to your PAYE code, then the simplest solution is simply to notify the tax office that deals with your employ
If you have spare personal allowances that are sufficient to absorb in full the extra earnings then you may not have any requirements at all (except that if you claim tax credits or other means-tested benefits then you would have to include it in the annual declaration for that purpose.
If this is a serious activity that amounts to self-employment, then you would have to register using the link mentioned earlier in this thread. However it may be misleading to suggest that there is no urgency. If you are under pensionable age, and the earnings are sufficient that you would be ineligible to claim exception from payment of class 2 National insurance contributions, then you risk a £100 penalty if you wait for more than 3 calendar months from the commencement of trade before registering.