The major changes Congress made last year in federal pension tax rules are so complicated and confusing that the Internal Revenue Service, which had to put them into effect, has just gotten around to telling people how the new system works.

Congress last year pulled the pension rug out from under federal and postal workers. It approved a backdated change in a longstanding benefit that previously allowed U.S. retirees (and others who contribute to their own pension plans) to recover their previously taxed contributions before their annuities were taxed. Before the change, the typical U.S. retiree enjoyed an 18-month period when his or her pension check was not taxed.

But Congress, pushed by the powerful House Ways and Means Committee, eliminated that so-called recovery period as part of the tax revision. Although the law wasn't signed until late last year, the provision affecting pensions was made retroactive to July 1986.

Tax revision also undid a major feature of a new benefit Congress had approved in mid-1986. It allowed U.S. retirees for the first time to elect to get lump sum payments equal to the amount of money they contributed toward their retirement. Generally speaking, those lump sum payments are worth about 7 percent of the total lifetime federal earnings of each individual.

As part of the new federal retirement law, retirees were given the option of taking a lump sum payment equal to their contributions. Although taking the payment would reduce the retirees' lifetime pension, Congress made them attractive to many by making the lump sum payments tax-free.

However, the tax overhaul law changed that. It subjected the lump sum payments to the same tax rules covering pensions. What that said is that part of one's pension is considered to come from one's own contributions and part (the lion's share) from the government's contributions. The latter, the law said, is taxable immediately. Only the prorated portion that came from the employe contribution was considered tax-free under the new rules.

Once federal workers got over their shock and rage at the change, those who were retiring or planning to retire then had to figure out what kind of taxes they were facing.

The Office of Personnel Management and the IRS had the unenviable task of translating what Congress had done. Making laws is a lot easier than interpreting them and making them work.

Guidelines were promised before income tax filing time, but the date came and went without them.

Well, the good news is that those official guidelines are finally here. They tell how to figure one's annuity or lump sum payment tax bite, which is based on estimated life span, estimated value of lifetime pension and a whole lot of other complicated factors. The information is contained in Publication No. 721, "Comprehensive Tax Guide to U.S. Civil Service Retirement Benefits." It is free from local IRS offices. The new guidelines (the May 1987 edition) replace earlier guidelines on federal pension taxes.

The bad news is that unless you are a tax lawyer or a blood relative of H&R Block, you will find the complex instructions, examples and actuarial tables tough going. But they are official, and they are finally out.

If the local IRS office doesn't have copies of the tax guide, District, Maryland and Virginia residents can write: IRS Forms Distribution Center, P.O. Box 25866, Richmond, VA 23260. Be sure to ask for the May 1987 edition of Form 721.