Let me suggest a DIY method that is superior to the string method. Perhaps you can get an inexpensive 4 foot aluminum level - the kind that are sold in places like Lowes and Home Depot. You can do a lot more with a level (or any straight piece of metal) than you can with string. Unfortunately the inferior string method is always mentioned when the subject of DIY alignment comes up.

Your 1st goal is to see if either your seat or down tube are leaning over to one side or the other. It can help if you have your bottom bracket cups without the axle screwed into your bottom bracket shell. These can be a truer data point than the face of your BB shell. Your level placed against these cups can allow you to measure any alignment discrepancies. A clamp to hold the level against the cup along side the seat tube can be helpful if you don't have 3 hands. Measure the distance from the level to the seat tube close to the BB shell. Now measure the distance of the seat tube up by the seat lug. Of course they should agree. If not then you should bend the seat tube until it is straight up. This alignment is important if your shoes are cleated into pedals to avoid aggravating your knees. If you can't do this then don't bother with a string method because that system only works (barely) if the front triangle is aligned. After bending the seat tube straight, now check the down tube the same way. Start with a measurement near the BB and then up by the head tube. Bend as necessary. Now you are ready to check the rear triangle.

Place your 4' level against the head tube and seat tube. It is best if your level is as close as possible to the down tube but not of course on a lug. Now measure the distance from the edge of the level to the inside face of the dropout. Flip the level to the other side and measure again. Of course they should agree or bend the chain stays until the dropouts are the same distance from the level as well as being whatever distance you require. 130mm?

There are various methods to check if your head tube is twisted including sighting along the head and seat tube to see if they look parallel. If you have a flat surface, you can try to get a 4 point touch with 4 blocks under the ends of both the seat and head tube.

It is possible to make your level a more useful alignment tool by taping a M5 hole at one end and using the bolt as a distance tool instead of a small ruler held against the level (to solve the problem of not having 3 hands available). The bolt can be improved by sharpening the end to a rounded point.

If all of this seem like too much, than of course you should take it to a frame builder that has all the right equipment and knowledge to do the job properly. The fork in particular can be very difficult to check the alignment without fixtures.