To take advantage of the new state tax break signed into law by Gov. Harry Hughes yesterday, Maryland Social Security recipients are advised to complete their federal returns first.

* se federal tax form 1040 to complete federal taxes. If federal taxes already have been computed on a simplified 1040A form, file an amended return 1040X, obtainable from the Internal Revenue Service.

* Figure Social Security tax calculation on the appropriate worksheet that is included with the 1040 packet.

* Carry the amount reported on line 21b of that federal form to line 61 on the back of the Maryland long form 502. Be sure to label line 61 as "Social Security income."

Persons who already have filed their state return may still amend it and qualify for the tax break.

Maryland officials recommended that they:

* Call or write to a state income tax office and request the amended state form 502X.

* Use the exact name and Social Security number written on the original state tax return and be sure to write the correct year, this time 1984, for the tax return.

* If the taxpayer cannot find an amended form 502X, send in a regular 502 return, but label it "amended" at the top.

Taxpayers who have questions should call any local state income tax offices or the comptroller of the Treasury's toll-free number, 1-800-MD TAXES.